diff --git a/README.md b/README.md
index 7617a01..1585016 100644
--- a/README.md
+++ b/README.md
@@ -311,11 +311,11 @@ Items not marked as completed may have partial implementations.
     - [ ] Compound - `and`, `or`
 - [ ] Mocks and Doubles
     - [ ] Mocks (Stub real types) - `mock TYPE { }`
-    - [ ] Doubles (Stand-ins for real types) - `double NAME { }`
-    - [ ] Method stubs - `allow().to receive()`, `allow().to receive().and_return()`
+    - [X] Doubles (Stand-ins for real types) - `double NAME { }`
+    - [X] Method stubs - `allow().to receive()`, `allow().to receive().and_return()`
     - [ ] Spies - `expect().to receive()`
-    - [ ] Message expectations - `expect().to receive().at_least()`
-    - [ ] Argument expectations - `expect().to receive().with()`
+    - [X] Message expectations - `expect().to receive().at_least()`
+    - [X] Argument expectations - `expect().to receive().with()`
     - [ ] Message ordering - `expect().to receive().ordered`
     - [ ] Null doubles
 - [ ] Runner
@@ -326,11 +326,12 @@ Items not marked as completed may have partial implementations.
     - [X] Dry run - for validation and checking formatted output
     - [X] Config block in `spec_helper.cr`
     - [X] Config file - `.spectator`
-- [X] Reporter and formatting
+- [ ] Reporter and formatting
     - [X] RSpec/Crystal Spec default
     - [X] JSON
     - [X] JUnit
     - [X] TAP
+    - [ ] HTML
 
 ### How it Works (in a nutshell)
 
diff --git a/src/spectator/anything.cr b/src/spectator/anything.cr
new file mode 100644
index 0000000..511a024
--- /dev/null
+++ b/src/spectator/anything.cr
@@ -0,0 +1,15 @@
+module Spectator
+  struct Anything
+    def ==(other)
+      true
+    end
+
+    def ===(other)
+      true
+    end
+
+    def =~(other)
+      true
+    end
+  end
+end
diff --git a/src/spectator/dsl/matchers.cr b/src/spectator/dsl/matchers.cr
index 5c55a06..0e779c3 100644
--- a/src/spectator/dsl/matchers.cr
+++ b/src/spectator/dsl/matchers.cr
@@ -683,6 +683,11 @@ module Spectator
       expect {{block}}.to raise_error({{type}}, {{message}})
     end
 
+    macro have_received(method)
+      %test_value = ::Spectator::TestValue.new(({{method.id.symbolize}}), {{method.id.stringify}})
+      ::Spectator::Matchers::ReceiveMatcher.new(%test_value)
+    end
+
     # Used to create predicate matchers.
     # Any missing method that starts with 'be_' or 'have_' will be handled.
     # All other method names will be ignored and raise a compile-time error.
diff --git a/src/spectator/dsl/mocks.cr b/src/spectator/dsl/mocks.cr
new file mode 100644
index 0000000..07d3c00
--- /dev/null
+++ b/src/spectator/dsl/mocks.cr
@@ -0,0 +1,173 @@
+require "../mocks"
+
+module Spectator::DSL
+  macro double(name = "Anonymous", **stubs, &block)
+    {% if name.is_a?(StringLiteral) %}
+      anonymous_double({{name}}, {{stubs.double_splat}})
+    {% else %}
+      {%
+        safe_name = name.id.symbolize.gsub(/\W/, "_").id
+        type_name = "Double#{safe_name}".id
+      %}
+
+      {% if block.is_a?(Nop) %}
+        create_double({{type_name}}, {{name}}, {{stubs.double_splat}})
+      {% else %}
+        define_double({{type_name}}, {{name}}, {{stubs.double_splat}}) {{block}}
+      {% end %}
+    {% end %}
+  end
+
+  macro create_double(type_name, name, **stubs)
+    {% type_name.resolve? || raise("Could not find a double labeled #{name}") %}
+
+    {{type_name}}.new.tap do |%double|
+      {% for name, value in stubs %}
+      allow(%double).to receive({{name.id}}).and_return({{value}})
+      {% end %}
+    end
+  end
+
+  macro define_double(type_name, name, **stubs, &block)
+    {% begin %}
+      {% if (name.is_a?(Path) || name.is_a?(Generic)) && (resolved = name.resolve?) %}
+        verify_double({{name}})
+        class {{type_name}} < ::Spectator::Mocks::VerifyingDouble(::{{resolved.id}})
+      {% else %}
+        class {{type_name}} < ::Spectator::Mocks::Double
+          def initialize(null = false)
+            super({{name.id.stringify}}, null)
+          end
+      {% end %}
+
+      def as_null_object
+        {{type_name}}.new(true)
+      end
+
+      # TODO: Do something with **stubs?
+
+      {{block.body}}
+    end
+    {% end %}
+  end
+
+  def anonymous_double(name = "Anonymous", **stubs)
+    Mocks::AnonymousDouble.new(name, stubs)
+  end
+
+  macro null_double(name, **stubs, &block)
+    {% if name.is_a?(StringLiteral) %}
+      anonymous_null_double({{name}}, {{stubs.double_splat}})
+    {% else %}
+      {%
+        safe_name = name.id.symbolize.gsub(/\W/, "_").id
+        type_name = "Double#{safe_name}".id
+      %}
+
+      {% if block.is_a?(Nop) %}
+        create_null_double({{type_name}}, {{name}}, {{stubs.double_splat}})
+      {% else %}
+        define_null_double({{type_name}}, {{name}}, {{stubs.double_splat}}) {{block}}
+      {% end %}
+    {% end %}
+  end
+
+  macro create_null_double(type_name, name, **stubs)
+    {% type_name.resolve? || raise("Could not find a double labeled #{name}") %}
+
+    {{type_name}}.new(true).tap do |%double|
+      {% for name, value in stubs %}
+      allow(%double).to receive({{name.id}}).and_return({{value}})
+      {% end %}
+    end
+  end
+
+  macro define_null_double(type_name, name, **stubs, &block)
+    class {{type_name}} < ::Spectator::Mocks::Double
+      def initialize(null = true)
+        super({{name.id.stringify}}, null)
+      end
+
+      def as_null_object
+        {{type_name}}.new(true)
+      end
+
+      # TODO: Do something with **stubs?
+
+      {{block.body}}
+    end
+  end
+
+  def anonymous_null_double(name = "Anonymous", **stubs)
+    AnonymousNullDouble.new(name, stubs)
+  end
+
+  macro mock(name, &block)
+    {% resolved = name.resolve
+       type = if resolved < Reference
+                :class
+              elsif resolved < Value
+                :struct
+              else
+                :module
+              end %}
+    {% begin %}
+      {{type.id}} ::{{resolved.id}}
+        include ::Spectator::Mocks::Stubs
+
+        {{block.body}}
+      end
+    {% end %}
+  end
+
+  macro verify_double(name, &block)
+    {% resolved = name.resolve
+       type = if resolved < Reference
+                :class
+              elsif resolved < Value
+                :struct
+              else
+                :module
+              end %}
+    {% begin %}
+      {{type.id}} ::{{resolved.id}}
+        include ::Spectator::Mocks::Reflection
+
+        macro finished
+          _spectator_reflect
+        end
+      end
+    {% end %}
+  end
+
+  def allow(thing)
+    Mocks::Allow.new(thing)
+  end
+
+  def allow_any_instance_of(type : T.class) forall T
+    Mocks::AllowAnyInstance(T).new
+  end
+
+  macro expect_any_instance_of(type, _source_file = __FILE__, _source_line = __LINE__)
+    %source = ::Spectator::Source.new({{_source_file}}, {{_source_line}})
+    ::Spectator::Mocks::ExpectAnyInstance({{type}}).new(%source)
+  end
+
+  macro receive(method_name, _source_file = __FILE__, _source_line = __LINE__, &block)
+    %source = ::Spectator::Source.new({{_source_file}}, {{_source_line}})
+    {% if block.is_a?(Nop) %}
+      ::Spectator::Mocks::NilMethodStub.new({{method_name.id.symbolize}}, %source)
+    {% else %}
+      ::Spectator::Mocks::ProcMethodStub.create({{method_name.id.symbolize}}, %source) { {{block.body}} }
+    {% end %}
+  end
+
+  macro receive_messages(_source_file = __FILE__, _source_line = __LINE__, **stubs)
+    %source = ::Spectator::Source.new({{_source_file}}, {{_source_line}})
+    %stubs = [] of ::Spectator::Mocks::MethodStub
+    {% for name, value in stubs %}
+    %stubs << ::Spectator::Mocks::ValueMethodStub.new({{name.id.symbolize}}, %source, {{value}})
+    {% end %}
+    %stubs
+  end
+end
diff --git a/src/spectator/dsl/values.cr b/src/spectator/dsl/values.cr
index 194e853..ea9fdf5 100644
--- a/src/spectator/dsl/values.cr
+++ b/src/spectator/dsl/values.cr
@@ -1,17 +1,17 @@
 module Spectator
   module DSL
     macro let(name, &block)
-      def %value
-        {{block.body}}
-      end
-
       @%wrapper : ::Spectator::ValueWrapper?
 
+      def {{name.id}}
+        {{block.body}}
+      end
+
       def {{name.id}}
         if (wrapper = @%wrapper)
-          wrapper.as(::Spectator::TypedValueWrapper(typeof(%value))).value
+          wrapper.as(::Spectator::TypedValueWrapper(typeof(previous_def))).value
         else
-          %value.tap do |value|
+          previous_def.tap do |value|
             @%wrapper = ::Spectator::TypedValueWrapper.new(value)
           end
         end
diff --git a/src/spectator/example_group.cr b/src/spectator/example_group.cr
index e5d35f8..ae8a5a8 100644
--- a/src/spectator/example_group.cr
+++ b/src/spectator/example_group.cr
@@ -35,6 +35,10 @@ module Spectator
       @example_count = children.sum(&.example_count)
     end
 
+    def double(id, sample_values)
+      @doubles[id].build(sample_values)
+    end
+
     getter context
 
     def initialize(@context : TestContext)
diff --git a/src/spectator/expectations/expectation_partial.cr b/src/spectator/expectations/expectation_partial.cr
index 8aa3709..b7a602d 100644
--- a/src/spectator/expectations/expectation_partial.cr
+++ b/src/spectator/expectations/expectation_partial.cr
@@ -24,6 +24,17 @@ module Spectator::Expectations
       report(match_data)
     end
 
+    def to(stub : Mocks::MethodStub) : Nil
+      Harness.current.mocks.expect(@actual.value, stub)
+      value = TestValue.new(stub.name, stub.to_s)
+      matcher = Matchers::ReceiveMatcher.new(value, stub.arguments?)
+      to_eventually(matcher)
+    end
+
+    def to(stubs : Enumerable(Mocks::MethodStub)) : Nil
+      stubs.each { |stub| to(stub) }
+    end
+
     # Asserts that some criteria defined by the matcher is not satisfied.
     # This is effectively the opposite of `#to`.
     def to_not(matcher) : Nil
@@ -31,6 +42,16 @@ module Spectator::Expectations
       report(match_data)
     end
 
+    def to_not(stub : Mocks::MethodStub) : Nil
+      value = TestValue.new(stub.name, stub.to_s)
+      matcher = Matchers::ReceiveMatcher.new(value, stub.arguments?)
+      to_never(matcher)
+    end
+
+    def to_not(stubs : Enumerable(Mocks::MethodStub)) : Nil
+      stubs.each { |stub| to_not(stub) }
+    end
+
     # ditto
     @[AlwaysInline]
     def not_to(matcher) : Nil
diff --git a/src/spectator/formatting/comment.cr b/src/spectator/formatting/comment.cr
index cbe6919..6a42685 100644
--- a/src/spectator/formatting/comment.cr
+++ b/src/spectator/formatting/comment.cr
@@ -13,7 +13,7 @@ module Spectator::Formatting
     end
 
     # Creates a colorized version of the comment.
-    def self.color(text : T) forall T
+    def self.color(text)
       Color.comment(new(text))
     end
   end
diff --git a/src/spectator/harness.cr b/src/spectator/harness.cr
index caa43a0..33ed2f0 100644
--- a/src/spectator/harness.cr
+++ b/src/spectator/harness.cr
@@ -1,3 +1,5 @@
+require "./mocks/registry"
+
 module Spectator
   # Helper class that acts as a gateway between example code and the test framework.
   # Every example must be invoked by passing it to `#run`.
@@ -34,6 +36,8 @@ module Spectator
     # Retrieves the current running example.
     getter example : Example
 
+    getter mocks : Mocks::Registry
+
     # Retrieves the group for the current running example.
     def group
       example.group
@@ -66,6 +70,7 @@ module Spectator
     # The example the harness is for should be passed in.
     private def initialize(@example)
       @reporter = Expectations::ExpectationReporter.new
+      @mocks = Mocks::Registry.new(@example.group.context)
       @deferred = Deque(->).new
     end
   end
diff --git a/src/spectator/includes.cr b/src/spectator/includes.cr
index c27fc49..4425f99 100644
--- a/src/spectator/includes.cr
+++ b/src/spectator/includes.cr
@@ -29,6 +29,8 @@ require "./example_group"
 require "./nested_example_group"
 require "./root_example_group"
 
+require "./mocks"
+
 require "./config"
 require "./config_builder"
 require "./config_source"
diff --git a/src/spectator/matchers/all_matcher.cr b/src/spectator/matchers/all_matcher.cr
index f413fbd..bf6231d 100644
--- a/src/spectator/matchers/all_matcher.cr
+++ b/src/spectator/matchers/all_matcher.cr
@@ -26,7 +26,7 @@ module Spectator::Matchers
         match_data = matcher.match(element)
         break match_data unless match_data.matched?
       end
-      found ? found : SuccessfulMatchData.new
+      found || SuccessfulMatchData.new
     end
 
     # Negated matching for this matcher is not supported.
diff --git a/src/spectator/matchers/change_from_matcher.cr b/src/spectator/matchers/change_from_matcher.cr
index cafb51d..fa3504b 100644
--- a/src/spectator/matchers/change_from_matcher.cr
+++ b/src/spectator/matchers/change_from_matcher.cr
@@ -63,12 +63,12 @@ module Spectator::Matchers
     end
 
     # Specifies what the resulting value of the expression must be.
-    def to(value : T) forall T
+    def to(value)
       ChangeExactMatcher.new(@expression, @expected, value)
     end
 
     # Specifies what the resulting value of the expression should change by.
-    def by(amount : T) forall T
+    def by(amount)
       ChangeExactMatcher.new(@expression, @expected, @expected + value)
     end
 
diff --git a/src/spectator/matchers/change_matcher.cr b/src/spectator/matchers/change_matcher.cr
index 2b016d5..5de2415 100644
--- a/src/spectator/matchers/change_matcher.cr
+++ b/src/spectator/matchers/change_matcher.cr
@@ -49,27 +49,27 @@ module Spectator::Matchers
     end
 
     # Specifies what the initial value of the expression must be.
-    def from(value : T) forall T
+    def from(value)
       ChangeFromMatcher.new(@expression, value)
     end
 
     # Specifies what the resulting value of the expression must be.
-    def to(value : T) forall T
+    def to(value)
       ChangeToMatcher.new(@expression, value)
     end
 
     # Specifies that t he resulting value must be some amount different.
-    def by(amount : T) forall T
+    def by(amount)
       ChangeRelativeMatcher.new(@expression, "by #{amount}") { |before, after| amount == after - before }
     end
 
     # Specifies that the resulting value must be at least some amount different.
-    def by_at_least(minimum : T) forall T
+    def by_at_least(minimum)
       ChangeRelativeMatcher.new(@expression, "by at least #{minimum}") { |before, after| minimum <= after - before }
     end
 
     # Specifies that the resulting value must be at most some amount different.
-    def by_at_most(maximum : T) forall T
+    def by_at_most(maximum)
       ChangeRelativeMatcher.new(@expression, "by at most #{maximum}") { |before, after| maximum >= after - before }
     end
 
diff --git a/src/spectator/matchers/change_to_matcher.cr b/src/spectator/matchers/change_to_matcher.cr
index a05587a..e29d23b 100644
--- a/src/spectator/matchers/change_to_matcher.cr
+++ b/src/spectator/matchers/change_to_matcher.cr
@@ -57,12 +57,12 @@ module Spectator::Matchers
     end
 
     # Specifies what the initial value of the expression must be.
-    def from(value : T) forall T
+    def from(value)
       ChangeExactMatcher.new(@expression, value, @expected)
     end
 
     # Specifies how much the initial value should change by.
-    def by(amount : T) forall T
+    def by(amount)
       ChangeExactMatcher.new(@expression, @expected - amount, @expected)
     end
 
diff --git a/src/spectator/matchers/failed_match_data.cr b/src/spectator/matchers/failed_match_data.cr
index 3e0857a..205b31c 100644
--- a/src/spectator/matchers/failed_match_data.cr
+++ b/src/spectator/matchers/failed_match_data.cr
@@ -14,6 +14,10 @@ module Spectator::Matchers
     # Additional information from the match that can be used to debug the problem.
     getter values : Array(Tuple(Symbol, String))
 
+    # Creates the match data.
+    def initialize(@failure_message, @values)
+    end
+
     # Creates the match data.
     def initialize(@failure_message, **values)
       @values = values.to_a
diff --git a/src/spectator/matchers/receive_matcher.cr b/src/spectator/matchers/receive_matcher.cr
new file mode 100644
index 0000000..9a96f82
--- /dev/null
+++ b/src/spectator/matchers/receive_matcher.cr
@@ -0,0 +1,126 @@
+require "../mocks"
+require "./standard_matcher"
+
+module Spectator::Matchers
+  struct ReceiveMatcher < StandardMatcher
+    alias Range = ::Range(Int32, Int32) | ::Range(Nil, Int32) | ::Range(Int32, Nil)
+
+    def initialize(@expected : TestExpression(Symbol), @args : Mocks::Arguments? = nil, @range : Range? = nil)
+    end
+
+    def description : String
+      range = @range
+      "received message #{@expected.label} #{range ? "#{humanize_range(range)} time(s)" : "At least once"} with #{@args || "any arguments"}"
+    end
+
+    def match?(actual : TestExpression(T)) : Bool forall T
+      calls = Harness.current.mocks.calls_for(actual.value, @expected.value)
+      calls.select! { |call| @args === call.args } if @args
+      if (range = @range)
+        range.includes?(calls.size)
+      else
+        !calls.empty?
+      end
+    end
+
+    def failure_message(actual : TestExpression(T)) : String forall T
+      range = @range
+      "#{actual.label} did not receive #{@expected.label} #{range ? "#{humanize_range(range)} time(s)" : "at least once"} with #{@args || "any arguments"}"
+    end
+
+    def failure_message_when_negated(actual) : String
+      range = @range
+      "#{actual.label} received #{@expected.label} #{range ? "#{humanize_range(range)} time(s)" : "at least once"} with #{@args || "any arguments"}"
+    end
+
+    def values(actual : TestExpression(T)) forall T
+      calls = Harness.current.mocks.calls_for(actual.value, @expected.value)
+      calls.select! { |call| @args === call.args } if @args
+      range = @range
+      {
+        expected: "#{range ? "#{humanize_range(range)} time(s)" : "At least once"} with #{@args || "any arguments"}",
+        received: "#{calls.size} time(s)",
+      }
+    end
+
+    def negated_values(actual : TestExpression(T)) forall T
+      calls = Harness.current.mocks.calls_for(actual.value, @expected.value)
+      calls.select! { |call| @args === call.args } if @args
+      range = @range
+      {
+        expected: "#{range ? "Not #{humanize_range(range)} time(s)" : "Never"} with #{@args || "any arguments"}",
+        received: "#{calls.size} time(s)",
+      }
+    end
+
+    def with(*args, **opts)
+      args = Mocks::GenericArguments.new(args, opts)
+      ReceiveMatcher.new(@expected, args, @range)
+    end
+
+    def once
+      ReceiveMatcher.new(@expected, @args, (1..1))
+    end
+
+    def twice
+      ReceiveMatcher.new(@expected, @args, (2..2))
+    end
+
+    def exactly(count)
+      Count.new(@expected, @args, (count..count))
+    end
+
+    def at_least(count)
+      Count.new(@expected, @args, (count..))
+    end
+
+    def at_most(count)
+      Count.new(@expected, @args, (..count))
+    end
+
+    def at_least_once
+      at_least(1).times
+    end
+
+    def at_least_twice
+      at_least(2).times
+    end
+
+    def at_most_once
+      at_most(1).times
+    end
+
+    def at_most_twice
+      at_most(2).times
+    end
+
+    def humanize_range(range : Range)
+      if (min = range.begin)
+        if (max = range.end)
+          if min == max
+            min
+          else
+            "#{min} to #{max}"
+          end
+        else
+          "At least #{min}"
+        end
+      else
+        if (max = range.end)
+          "At most #{max}"
+        else
+          raise "Unexpected endless range"
+        end
+      end
+    end
+
+    private struct Count
+      def initialize(@expected : TestExpression(Symbol), @args : Mocks::Arguments?, @range : Range)
+      end
+
+      def times
+        ReceiveMatcher.new(@expected, @args, @range)
+      end
+    end
+  end
+end
diff --git a/src/spectator/matchers/receive_type_matcher.cr b/src/spectator/matchers/receive_type_matcher.cr
new file mode 100644
index 0000000..c716f05
--- /dev/null
+++ b/src/spectator/matchers/receive_type_matcher.cr
@@ -0,0 +1,126 @@
+require "../mocks"
+require "./standard_matcher"
+
+module Spectator::Matchers
+  struct ReceiveTypeMatcher < StandardMatcher
+    alias Range = ::Range(Int32, Int32) | ::Range(Nil, Int32) | ::Range(Int32, Nil)
+
+    def initialize(@expected : TestExpression(Symbol), @args : Mocks::Arguments? = nil, @range : Range? = nil)
+    end
+
+    def description : String
+      range = @range
+      "received message #{@expected.label} #{range ? "#{humanize_range(range)} time(s)" : "At least once"} with #{@args || "any arguments"}"
+    end
+
+    def match?(actual : TestExpression(T)) : Bool forall T
+      calls = Harness.current.mocks.calls_for_type(actual.value, @expected.value)
+      calls.select! { |call| @args === call.args } if @args
+      if (range = @range)
+        range.includes?(calls.size)
+      else
+        !calls.empty?
+      end
+    end
+
+    def failure_message(actual : TestExpression(T)) : String forall T
+      range = @range
+      "#{actual.label} did not receive #{@expected.label} #{range ? "#{humanize_range(range)} time(s)" : "at least once"} with #{@args || "any arguments"}"
+    end
+
+    def failure_message_when_negated(actual : TestExpression(T)) : String forall T
+      range = @range
+      "#{actual.label} received #{@expected.label} #{range ? "#{humanize_range(range)} time(s)" : "at least once"} with #{@args || "any arguments"}"
+    end
+
+    def values(actual : TestExpression(T)) forall T
+      calls = Harness.current.mocks.calls_for_type(T, @expected.value)
+      calls.select! { |call| @args === call.args } if @args
+      range = @range
+      {
+        expected: "#{range ? "#{humanize_range(range)} time(s)" : "At least once"} with #{@args || "any arguments"}",
+        received: "#{calls.size} time(s)",
+      }
+    end
+
+    def negated_values(actual : TestExpression(T)) forall T
+      calls = Harness.current.mocks.calls_for_type(T, @expected.value)
+      calls.select! { |call| @args === call.args } if @args
+      range = @range
+      {
+        expected: "#{range ? "Not #{humanize_range(range)} time(s)" : "Never"} with #{@args || "any arguments"}",
+        received: "#{calls.size} time(s)",
+      }
+    end
+
+    def with(*args, **opts)
+      args = Mocks::GenericArguments.new(args, opts)
+      ReceiveTypeMatcher.new(@expected, args, @range)
+    end
+
+    def once
+      ReceiveTypeMatcher.new(@expected, @args, (1..1))
+    end
+
+    def twice
+      ReceiveTypeMatcher.new(@expected, @args, (2..2))
+    end
+
+    def exactly(count)
+      Count.new(@expected, @args, (count..count))
+    end
+
+    def at_least(count)
+      Count.new(@expected, @args, (count..))
+    end
+
+    def at_most(count)
+      Count.new(@expected, @args, (..count))
+    end
+
+    def at_least_once
+      at_least(1).times
+    end
+
+    def at_least_twice
+      at_least(2).times
+    end
+
+    def at_most_once
+      at_most(1).times
+    end
+
+    def at_most_twice
+      at_most(2).times
+    end
+
+    def humanize_range(range : Range)
+      if (min = range.begin)
+        if (max = range.end)
+          if min == max
+            min
+          else
+            "#{min} to #{max}"
+          end
+        else
+          "At least #{min}"
+        end
+      else
+        if (max = range.end)
+          "At most #{max}"
+        else
+          raise "Unexpected endless range"
+        end
+      end
+    end
+
+    private struct Count
+      def initialize(@expected : TestExpression(Symbol), @args : Mocks::Arguments?, @range : Range)
+      end
+
+      def times
+        ReceiveTypeMatcher.new(@expected, @args, @range)
+      end
+    end
+  end
+end
diff --git a/src/spectator/matchers/standard_matcher.cr b/src/spectator/matchers/standard_matcher.cr
index 414e491..2a01ab3 100644
--- a/src/spectator/matchers/standard_matcher.cr
+++ b/src/spectator/matchers/standard_matcher.cr
@@ -27,7 +27,7 @@ module Spectator::Matchers
       if match?(actual)
         SuccessfulMatchData.new
       else
-        FailedMatchData.new(failure_message(actual), **values(actual))
+        FailedMatchData.new(failure_message(actual), values(actual).to_a)
       end
     end
 
@@ -42,7 +42,7 @@ module Spectator::Matchers
       if does_not_match?(actual)
         SuccessfulMatchData.new
       else
-        FailedMatchData.new(failure_message_when_negated(actual), **negated_values(actual))
+        FailedMatchData.new(failure_message_when_negated(actual), negated_values(actual).to_a)
       end
     end
 
@@ -66,7 +66,7 @@ module Spectator::Matchers
     # The message should typically only contain the test expression labels.
     # Actual values should be returned by `#values`.
     private def failure_message_when_negated(actual : TestExpression(T)) : String forall T
-      {% raise "Negation with #{@type.name} is not supported." %}
+      raise "Negation with #{self.class} is not supported."
     end
 
     # Checks whether the matcher is satisifed with the expression given to it.
diff --git a/src/spectator/matchers/truthy_matcher.cr b/src/spectator/matchers/truthy_matcher.cr
index df21d8c..bc26a6e 100644
--- a/src/spectator/matchers/truthy_matcher.cr
+++ b/src/spectator/matchers/truthy_matcher.cr
@@ -27,7 +27,7 @@ module Spectator::Matchers
     # ```
     # expect(0).to be < 1
     # ```
-    def <(value : ExpectedType) forall ExpectedType
+    def <(value)
       expected = TestValue.new(value)
       LessThanMatcher.new(expected)
     end
@@ -37,7 +37,7 @@ module Spectator::Matchers
     # ```
     # expect(0).to be <= 1
     # ```
-    def <=(value : ExpectedType) forall ExpectedType
+    def <=(value)
       expected = TestValue.new(value)
       LessThanEqualMatcher.new(expected)
     end
@@ -47,7 +47,7 @@ module Spectator::Matchers
     # ```
     # expect(2).to be > 1
     # ```
-    def >(value : ExpectedType) forall ExpectedType
+    def >(value)
       expected = TestValue.new(value)
       GreaterThanMatcher.new(expected)
     end
@@ -57,7 +57,7 @@ module Spectator::Matchers
     # ```
     # expect(2).to be >= 1
     # ```
-    def >=(value : ExpectedType) forall ExpectedType
+    def >=(value)
       expected = TestValue.new(value)
       GreaterThanEqualMatcher.new(expected)
     end
@@ -67,7 +67,7 @@ module Spectator::Matchers
     # ```
     # expect(0).to be == 0
     # ```
-    def ==(value : ExpectedType) forall ExpectedType
+    def ==(value)
       expected = TestValue.new(value)
       EqualityMatcher.new(expected)
     end
@@ -77,7 +77,7 @@ module Spectator::Matchers
     # ```
     # expect(0).to be != 1
     # ```
-    def !=(value : ExpectedType) forall ExpectedType
+    def !=(value)
       expected = TestValue.new(value)
       InequalityMatcher.new(expected)
     end
@@ -114,7 +114,7 @@ module Spectator::Matchers
       {
         expected: @truthy ? "Not false or nil" : "false or nil",
         actual:   actual.value.inspect,
-        truthy:   !!actual.value.inspect,
+        truthy:   (!!actual.value).inspect,
       }
     end
 
@@ -124,7 +124,7 @@ module Spectator::Matchers
       {
         expected: @truthy ? "false or nil" : "Not false or nil",
         actual:   actual.value.inspect,
-        truthy:   !!actual.value.inspect,
+        truthy:   (!!actual.value).inspect,
       }
     end
 
diff --git a/src/spectator/mocks.cr b/src/spectator/mocks.cr
new file mode 100644
index 0000000..d36eeff
--- /dev/null
+++ b/src/spectator/mocks.cr
@@ -0,0 +1,13 @@
+require "./mocks/*"
+
+module Spectator
+  # Functionality for mocking existing types.
+  module Mocks
+    def self.run(context : TestContext)
+      Registry.prepare(context)
+      yield
+    ensure
+      Registry.reset
+    end
+  end
+end
diff --git a/src/spectator/mocks/allow.cr b/src/spectator/mocks/allow.cr
new file mode 100644
index 0000000..fb6b86f
--- /dev/null
+++ b/src/spectator/mocks/allow.cr
@@ -0,0 +1,18 @@
+require "./registry"
+
+module Spectator::Mocks
+  struct Allow(T)
+    def initialize(@mock : T)
+    end
+
+    def to(stub : MethodStub) : Nil
+      Harness.current.mocks.add_stub(@mock, stub)
+    end
+
+    def to(stubs : Enumerable(MethodStub)) : Nil
+      stubs.each do |stub|
+        Harness.current.mocks.add_stub(@mock, stub)
+      end
+    end
+  end
+end
diff --git a/src/spectator/mocks/allow_any_instance.cr b/src/spectator/mocks/allow_any_instance.cr
new file mode 100644
index 0000000..b652940
--- /dev/null
+++ b/src/spectator/mocks/allow_any_instance.cr
@@ -0,0 +1,15 @@
+require "./registry"
+
+module Spectator::Mocks
+  struct AllowAnyInstance(T)
+    def to(stub : MethodStub) : Nil
+      Harness.current.mocks.add_type_stub(T, stub)
+    end
+
+    def to(stubs : Enumerable(MethodStub)) : Nil
+      stubs.each do |stub|
+        Harness.current.mocks.add_type_stub(T, stub)
+      end
+    end
+  end
+end
diff --git a/src/spectator/mocks/anonymous_double.cr b/src/spectator/mocks/anonymous_double.cr
new file mode 100644
index 0000000..439901a
--- /dev/null
+++ b/src/spectator/mocks/anonymous_double.cr
@@ -0,0 +1,26 @@
+module Spectator::Mocks
+  class AnonymousDouble(T)
+    def initialize(@name : String, @values : T)
+    end
+
+    def as_null_object
+      AnonymousNullDouble.new(@name, @values)
+    end
+
+    macro method_missing(call)
+      args = ::Spectator::Mocks::GenericArguments.create({{call.args.splat}})
+      call = ::Spectator::Mocks::GenericMethodCall.new({{call.name.symbolize}}, args)
+      ::Spectator::Harness.current.mocks.record_call(self, call)
+      if (stub = ::Spectator::Harness.current.mocks.find_stub(self, call))
+        stub.call!(args) do
+          @values.fetch({{call.name.symbolize}}) { raise "Consistency error - method stubbed with no implementation"; nil }
+        end
+      else
+        @values.fetch({{call.name.symbolize}}) do
+          return nil if ::Spectator::Harness.current.mocks.expected?(self, call)
+          raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{call.name}}")
+        end
+      end
+    end
+  end
+end
diff --git a/src/spectator/mocks/anonymous_null_double.cr b/src/spectator/mocks/anonymous_null_double.cr
new file mode 100644
index 0000000..8fa338c
--- /dev/null
+++ b/src/spectator/mocks/anonymous_null_double.cr
@@ -0,0 +1,17 @@
+module Spectator::Mocks
+  class AnonymousNullDouble(T)
+    def initialize(@name : String, @values : T)
+    end
+
+    macro method_missing(call)
+      args = ::Spectator::Mocks::GenericArguments.create({{call.args.splat}})
+      call = ::Spectator::Mocks::GenericMethodCall.new({{call.name.symbolize}}, args)
+      ::Spectator::Harness.current.mocks.record_call(self, call)
+      if (stub = ::Spectator::Harness.current.mocks.find_stub(self, call))
+        stub.call!(args) { @values.fetch({{call.name.symbolize}}) { self } }
+      else
+        @values.fetch({{call.name.symbolize}}) { self }
+      end
+    end
+  end
+end
diff --git a/src/spectator/mocks/arguments.cr b/src/spectator/mocks/arguments.cr
new file mode 100644
index 0000000..e909235
--- /dev/null
+++ b/src/spectator/mocks/arguments.cr
@@ -0,0 +1,4 @@
+module Spectator::Mocks
+  abstract class Arguments
+  end
+end
diff --git a/src/spectator/mocks/double.cr b/src/spectator/mocks/double.cr
new file mode 100644
index 0000000..de218ba
--- /dev/null
+++ b/src/spectator/mocks/double.cr
@@ -0,0 +1,98 @@
+require "./generic_method_call"
+require "./generic_method_stub"
+require "./unexpected_message_error"
+
+module Spectator::Mocks
+  abstract class Double
+    def initialize(@spectator_double_name : String, @null = false)
+    end
+
+    private macro stub(definition, &block)
+      {%
+        name = nil
+        params = nil
+        args = nil
+        body = nil
+        if definition.is_a?(Call) # stub foo { :bar }
+          named = false
+          name = definition.name.id
+          params = definition.args
+          args = params.map do |p|
+            n = p.is_a?(TypeDeclaration) ? p.var : p.id
+            r = named ? "#{n}: #{n}".id : n
+            named = true if n.starts_with?('*')
+            r
+          end
+          body = definition.block.is_a?(Nop) ? block : definition.block
+        elsif definition.is_a?(TypeDeclaration) # stub foo : Symbol
+          name = definition.var
+          params = [] of MacroId
+          args = [] of MacroId
+          body = block
+        else
+          raise "Unrecognized stub format"
+        end
+      %}
+
+      def {{name}}({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        %args = ::Spectator::Mocks::GenericArguments.create({{args.splat}})
+        %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+        ::Spectator::Harness.current.mocks.record_call(self, %call)
+        if (%stub = ::Spectator::Harness.current.mocks.find_stub(self, %call))
+          %stub.call!(%args) { %method({{args.splat}}) }
+        else
+          %method({{args.splat}})
+        end
+      end
+
+      def {{name}}({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        %args = ::Spectator::Mocks::GenericArguments.create({{args.splat}})
+        %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+        ::Spectator::Harness.current.mocks.record_call(self, %call)
+        if (%stub = ::Spectator::Harness.current.mocks.find_stub(self, %call))
+          %stub.call!(%args) { %method({{args.splat}}) { |*%ya| yield *%ya } }
+        else
+          %method({{args.splat}}) do |*%yield_args|
+            yield *%yield_args
+          end
+        end
+      end
+
+      def %method({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        {% if body && !body.is_a?(Nop) %}
+          {{body.body}}
+        {% else %}
+          %args = ::Spectator::Mocks::GenericArguments.create({{params.splat}})
+          %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+          unless ::Spectator::Harness.current.mocks.expected?(self, %call)
+            raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{name}}")
+          end
+
+          # This code shouldn't be reached, but makes the compiler happy to have a matching return type.
+          {% if definition.is_a?(TypeDeclaration) %}
+            %x = uninitialized {{definition.type}}
+          {% else %}
+            nil
+          {% end %}
+        {% end %}
+      end
+    end
+
+    macro method_missing(call)
+      args = ::Spectator::Mocks::GenericArguments.create({{call.args.splat}})
+      call = ::Spectator::Mocks::GenericMethodCall.new({{call.name.symbolize}}, args)
+      ::Spectator::Harness.current.mocks.record_call(self, call)
+
+      return self if @null
+      return self if ::Spectator::Harness.current.mocks.expected?(self, call)
+
+      raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{call.name}}")
+    end
+
+    def to_s(io)
+      io << "Double("
+      io << @spectator_double_name
+      io << ')'
+    end
+  end
+end
diff --git a/src/spectator/mocks/exception_method_stub.cr b/src/spectator/mocks/exception_method_stub.cr
new file mode 100644
index 0000000..96342bc
--- /dev/null
+++ b/src/spectator/mocks/exception_method_stub.cr
@@ -0,0 +1,14 @@
+require "./generic_arguments"
+require "./generic_method_stub"
+
+module Spectator::Mocks
+  class ExceptionMethodStub(ExceptionType) < GenericMethodStub(Nil)
+    def initialize(name, source, @exception : ExceptionType, args = nil)
+      super(name, source, args)
+    end
+
+    def call(_args : GenericArguments(T, NT), &_original : -> RT) forall T, NT, RT
+      raise @exception
+    end
+  end
+end
diff --git a/src/spectator/mocks/expect_any_instance.cr b/src/spectator/mocks/expect_any_instance.cr
new file mode 100644
index 0000000..c51c0e8
--- /dev/null
+++ b/src/spectator/mocks/expect_any_instance.cr
@@ -0,0 +1,23 @@
+require "./registry"
+
+module Spectator::Mocks
+  struct ExpectAnyInstance(T)
+    def initialize(@source : Source)
+    end
+
+    def to(stub : MethodStub) : Nil
+      actual = TestValue.new(T)
+      Harness.current.mocks.expect(T, stub.name)
+      value = TestValue.new(stub.name, stub.to_s)
+      matcher = Matchers::ReceiveTypeMatcher.new(value, stub.arguments?)
+      partial = Expectations::ExpectationPartial.new(actual, @source)
+      partial.to_eventually(matcher)
+    end
+
+    def to(stubs : Enumerable(MethodStub)) : Nil
+      stubs.each do |stub|
+        to(stub)
+      end
+    end
+  end
+end
diff --git a/src/spectator/mocks/generic_arguments.cr b/src/spectator/mocks/generic_arguments.cr
new file mode 100644
index 0000000..18d4193
--- /dev/null
+++ b/src/spectator/mocks/generic_arguments.cr
@@ -0,0 +1,42 @@
+require "./arguments"
+
+module Spectator::Mocks
+  class GenericArguments(T, NT) < Arguments
+    protected getter args
+    protected getter opts
+
+    def initialize(@args : T, @opts : NT)
+    end
+
+    def self.create(*args, **opts)
+      GenericArguments.new(args, opts)
+    end
+
+    def pass_to(dispatcher)
+      dispatcher.call(*@args, **@opts)
+    end
+
+    def ===(other) : Bool
+      return false unless @args === other.args
+      return false unless @opts.size === other.opts.size
+
+      @opts.keys.all? do |key|
+        other.opts.has_key?(key) && @opts[key] === other.opts[key]
+      end
+    end
+
+    def to_s(io)
+      @args.each_with_index do |arg, i|
+        arg.inspect(io)
+        io << ", " if i < @args.size - 1
+      end
+      io << ", " unless @args.empty? || @opts.empty?
+      @opts.each_with_index do |key, value, i|
+        io << key
+        io << ": "
+        value.inspect(io)
+        io << ", " if i < @opts.size - 1
+      end
+    end
+  end
+end
diff --git a/src/spectator/mocks/generic_method_call.cr b/src/spectator/mocks/generic_method_call.cr
new file mode 100644
index 0000000..64f8999
--- /dev/null
+++ b/src/spectator/mocks/generic_method_call.cr
@@ -0,0 +1,19 @@
+require "./generic_arguments"
+require "./method_call"
+
+module Spectator::Mocks
+  class GenericMethodCall(T, NT) < MethodCall
+    getter args
+
+    def initialize(name : Symbol, @args : GenericArguments(T, NT))
+      super(name)
+    end
+
+    def to_s(io)
+      super
+      io << '('
+      io << @args
+      io << ')'
+    end
+  end
+end
diff --git a/src/spectator/mocks/generic_method_stub.cr b/src/spectator/mocks/generic_method_stub.cr
new file mode 100644
index 0000000..c0ee38f
--- /dev/null
+++ b/src/spectator/mocks/generic_method_stub.cr
@@ -0,0 +1,31 @@
+require "./arguments"
+require "./generic_arguments"
+require "./method_call"
+require "./method_stub"
+
+module Spectator::Mocks
+  abstract class GenericMethodStub(ReturnType) < MethodStub
+    getter! arguments : Arguments
+
+    def initialize(name, source, @args : Arguments? = nil)
+      super(name, source)
+    end
+
+    def callable?(call : GenericMethodCall(T, NT)) : Bool forall T, NT
+      super && (!@args || @args === call.args)
+    end
+
+    def to_s(io)
+      super(io)
+      if @args
+        io << '('
+        io << @args
+        io << ')'
+      end
+      io << " : "
+      io << ReturnType
+      io << " at "
+      io << @source
+    end
+  end
+end
diff --git a/src/spectator/mocks/method_call.cr b/src/spectator/mocks/method_call.cr
new file mode 100644
index 0000000..00c6d0c
--- /dev/null
+++ b/src/spectator/mocks/method_call.cr
@@ -0,0 +1,13 @@
+module Spectator::Mocks
+  abstract class MethodCall
+    getter name : Symbol
+
+    def initialize(@name : Symbol)
+    end
+
+    def to_s(io)
+      io << '#'
+      io << @name
+    end
+  end
+end
diff --git a/src/spectator/mocks/method_stub.cr b/src/spectator/mocks/method_stub.cr
new file mode 100644
index 0000000..245bff3
--- /dev/null
+++ b/src/spectator/mocks/method_stub.cr
@@ -0,0 +1,33 @@
+require "../source"
+require "./generic_method_call"
+
+module Spectator::Mocks
+  abstract class MethodStub
+    getter name : Symbol
+
+    getter source : Source
+
+    def initialize(@name, @source)
+    end
+
+    def callable?(call : GenericMethodCall(T, NT)) : Bool forall T, NT
+      @name == call.name
+    end
+
+    abstract def call(args : GenericArguments(T, NT), &original : -> RT) forall T, NT, RT
+
+    def call!(args : GenericArguments(T, NT), &original : -> RT) : RT forall T, NT, RT
+      value = call(args, &original)
+      if value.is_a?(RT)
+        value.as(RT)
+      else
+        raise TypeCastError.new("The return type of stub #{self} doesn't match the expected type #{RT}")
+      end
+    end
+
+    def to_s(io)
+      io << '#'
+      io << @name
+    end
+  end
+end
diff --git a/src/spectator/mocks/multi_value_method_stub.cr b/src/spectator/mocks/multi_value_method_stub.cr
new file mode 100644
index 0000000..c623a4c
--- /dev/null
+++ b/src/spectator/mocks/multi_value_method_stub.cr
@@ -0,0 +1,19 @@
+require "./generic_arguments"
+require "./generic_method_stub"
+
+module Spectator::Mocks
+  class MultiValueMethodStub(ReturnType) < GenericMethodStub(ReturnType)
+    @index = 0
+
+    def initialize(name, source, @values : ReturnType, args = nil)
+      super(name, source, args)
+      raise ArgumentError.new("Values must have at least one item") if @values.size < 1
+    end
+
+    def call(_args : GenericArguments(T, NT), &_original : -> RT) forall T, NT, RT
+      value = @values[@index]
+      @index += 1 if @index < @values.size - 1
+      value
+    end
+  end
+end
diff --git a/src/spectator/mocks/nil_method_stub.cr b/src/spectator/mocks/nil_method_stub.cr
new file mode 100644
index 0000000..592d3b6
--- /dev/null
+++ b/src/spectator/mocks/nil_method_stub.cr
@@ -0,0 +1,48 @@
+require "./generic_arguments"
+require "./generic_method_stub"
+require "./value_method_stub"
+
+module Spectator::Mocks
+  class NilMethodStub < GenericMethodStub(Nil)
+    def call(_args : GenericArguments(T, NT), &_original : -> RT) forall T, NT, RT
+      nil
+    end
+
+    def and_return
+      self
+    end
+
+    def and_return(value)
+      ValueMethodStub.new(@name, @source, value, @args)
+    end
+
+    def and_return(*values)
+      MultiValueMethodStub.new(@name, @source, values.to_a, @args)
+    end
+
+    def and_raise(exception_type : Exception.class)
+      ExceptionMethodStub.new(@name, @source, exception_type.new, @args)
+    end
+
+    def and_raise(exception : Exception)
+      ExceptionMethodStub.new(@name, @source, exception, @args)
+    end
+
+    def and_raise(message : String)
+      ExceptionMethodStub.new(@name, @source, Exception.new(message), @args)
+    end
+
+    def and_raise(exception_type : Exception.class, *args) forall T
+      ExceptionMethodStub.new(@name, @source, exception_type.new(*args), @args)
+    end
+
+    def with(*args : *T, **opts : **NT) forall T, NT
+      args = GenericArguments.new(args, opts)
+      NilMethodStub.new(@name, @source, args)
+    end
+
+    def and_call_original
+      OriginalMethodStub.new(@name, @source, @args)
+    end
+  end
+end
diff --git a/src/spectator/mocks/original_method_stub.cr b/src/spectator/mocks/original_method_stub.cr
new file mode 100644
index 0000000..2097eb6
--- /dev/null
+++ b/src/spectator/mocks/original_method_stub.cr
@@ -0,0 +1,10 @@
+require "./generic_arguments"
+require "./generic_method_stub"
+
+module Spectator::Mocks
+  class OriginalMethodStub < GenericMethodStub(Nil)
+    def call(_args : GenericArguments(T, NT), &_original : -> RT) forall T, NT, RT
+      yield
+    end
+  end
+end
diff --git a/src/spectator/mocks/proc_method_stub.cr b/src/spectator/mocks/proc_method_stub.cr
new file mode 100644
index 0000000..1d55db3
--- /dev/null
+++ b/src/spectator/mocks/proc_method_stub.cr
@@ -0,0 +1,18 @@
+require "./arguments"
+require "./generic_method_stub"
+
+module Spectator::Mocks
+  class ProcMethodStub(ReturnType) < GenericMethodStub(ReturnType)
+    def initialize(name, source, @proc : -> ReturnType, args = nil)
+      super(name, source, args)
+    end
+
+    def self.create(name, source, args = nil, &block : -> T) forall T
+      ProcMethodStub.new(name, source, block, args)
+    end
+
+    def call(_args : GenericArguments(T, NT), &_original : -> RT) forall T, NT, RT
+      @proc.call
+    end
+  end
+end
diff --git a/src/spectator/mocks/reflection.cr b/src/spectator/mocks/reflection.cr
new file mode 100644
index 0000000..43f20f2
--- /dev/null
+++ b/src/spectator/mocks/reflection.cr
@@ -0,0 +1,21 @@
+require "../anything"
+
+module Spectator::Mocks
+  module Reflection
+    private macro _spectator_reflect
+      {% for meth in @type.methods %}
+        %source = ::Spectator::Source.new({{meth.filename}}, {{meth.line_number}})
+        %args = ::Spectator::Mocks::GenericArguments.create(
+          {% for arg, i in meth.args %}
+            {% if meth.splat_index && i == meth.splat_index %}
+              *{{arg.restriction || "::Spectator::Anything.new".id}}{% if i < meth.args.size %},{% end %}
+            {% else %}
+              {{arg.restriction || "::Spectator::Anything.new".id}}{% if i < meth.args.size %},{% end %}
+            {% end %}
+          {% end %}
+        )
+        ::Spectator::Mocks::TypeRegistry.add({{@type.id.stringify}}, {{meth.name.symbolize}}, %source, %args)
+      {% end %}
+    end
+  end
+end
diff --git a/src/spectator/mocks/registry.cr b/src/spectator/mocks/registry.cr
new file mode 100644
index 0000000..a946ae4
--- /dev/null
+++ b/src/spectator/mocks/registry.cr
@@ -0,0 +1,107 @@
+module Spectator::Mocks
+  class Registry
+    alias Key = Tuple(String, UInt64)
+
+    private struct Entry
+      getter stubs = Deque(MethodStub).new
+      getter calls = Deque(MethodCall).new
+      getter expected = Set(MethodStub).new
+    end
+
+    @all_instances = {} of String => Entry
+    @entries = {} of Key => Entry
+
+    def initialize(context : TestContext)
+      current_context = context
+      while current_context
+        current_context.stubs.each do |k, v|
+          stubs = if @all_instances.has_key?(k)
+                    @all_instances[k].stubs
+                  else
+                    entry = Entry.new
+                    @all_instances[k] = entry
+                    entry.stubs
+                  end
+          stubs.concat(v)
+        end
+        current_context = current_context.parent?
+      end
+    end
+
+    def reset : Nil
+      @entries.clear
+    end
+
+    def add_stub(object, stub : MethodStub) : Nil
+      # Stubs are added in reverse order,
+      # so that later-defined stubs override previously defined ones.
+      fetch_instance(object).stubs.unshift(stub)
+    end
+
+    def add_type_stub(type, stub : MethodStub) : Nil
+      fetch_type(type).stubs.unshift(stub)
+    end
+
+    def stubbed?(object, method_name : Symbol) : Bool
+      fetch_instance(object).stubs.any? { |stub| stub.name == method_name } ||
+        fetch_type(object.class).stubs.any? { |stub| stub.name == method_name }
+    end
+
+    def find_stub(object, call : GenericMethodCall(T, NT)) forall T, NT
+      fetch_instance(object).stubs.find(&.callable?(call)) ||
+        fetch_type(object.class).stubs.find(&.callable?(call))
+    end
+
+    def record_call(object, call : MethodCall) : Nil
+      fetch_instance(object).calls << call
+      fetch_type(object.class).calls << call
+    end
+
+    def calls_for(object, method_name : Symbol)
+      fetch_instance(object).calls.select { |call| call.name == method_name }
+    end
+
+    def calls_for_type(type : T.class, method_name : Symbol) forall T
+      fetch_type(type).calls.select { |call| call.name == method_name }
+    end
+
+    def expected?(object, call : GenericMethodCall(T, NT)) : Bool forall T, NT
+      fetch_instance(object).expected.any?(&.callable?(call)) ||
+        fetch_type(object.class).expected.any?(&.callable?(call))
+    end
+
+    def expect(object, stub : MethodStub) : Nil
+      fetch_instance(object).expected.add(stub)
+    end
+
+    def expect(type : T.class, stub : MethodStub) : Nil forall T
+      fetch_type(type).expected.add(stub)
+    end
+
+    private def fetch_instance(object)
+      key = unique_key(object)
+      if @entries.has_key?(key)
+        @entries[key]
+      else
+        @entries[key] = Entry.new
+      end
+    end
+
+    private def fetch_type(type)
+      key = type.name
+      if @all_instances.has_key?(key)
+        @all_instances[key]
+      else
+        @all_instances[key] = Entry.new
+      end
+    end
+
+    private def unique_key(reference : Reference)
+      {reference.class.name, reference.object_id}
+    end
+
+    private def unique_key(value : Value)
+      {value.class.name, value.hash}
+    end
+  end
+end
diff --git a/src/spectator/mocks/stubs.cr b/src/spectator/mocks/stubs.cr
new file mode 100644
index 0000000..089e637
--- /dev/null
+++ b/src/spectator/mocks/stubs.cr
@@ -0,0 +1,80 @@
+module Spectator::Mocks
+  module Stubs
+    private macro stub(definition, _file = __FILE__, _line = __LINE__, &block)
+      {%
+        receiver = nil
+        name = nil
+        params = nil
+        args = nil
+        body = nil
+        if definition.is_a?(Call) # stub foo { :bar }
+          receiver = definition.receiver.id
+          named = false
+          name = definition.name.id
+          params = definition.args
+          args = params.map do |p|
+            n = p.is_a?(TypeDeclaration) ? p.var : p.id
+            r = named ? "#{n}: #{n}".id : n
+            named = true if n.starts_with?('*')
+            r
+          end
+          body = definition.block.is_a?(Nop) ? block : definition.block
+        elsif definition.is_a?(TypeDeclaration) # stub foo : Symbol
+          name = definition.var
+          params = [] of MacroId
+          args = [] of MacroId
+          body = block
+        else
+          raise "Unrecognized stub format"
+        end
+
+        original = if @type.methods.find { |m| m.name.id == name }
+                     :previous_def
+                   else
+                     :super
+                   end.id
+        receiver = if receiver == :self.id
+                     original = :previous_def.id
+                     "self."
+                   else
+                     ""
+                   end.id
+      %}
+
+      {% if body && !body.is_a?(Nop) %}
+        %source = ::Spectator::Source.new({{_file}}, {{_line}})
+        %proc = ->{
+          {{body.body}}
+        }
+        %ds = ::Spectator::Mocks::ProcMethodStub.new({{name.symbolize}}, %source, %proc)
+        ::Spectator::SpecBuilder.add_default_stub({{@type.name}}, %ds)
+      {% end %}
+
+      def {{receiver}}{{name}}({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        if (%harness = ::Spectator::Harness.current?)
+          %args = ::Spectator::Mocks::GenericArguments.create({{args.splat}})
+          %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+          %harness.mocks.record_call(self, %call)
+          if (%stub = %harness.mocks.find_stub(self, %call))
+            return %stub.call!(%args) { {{original}}({{args.splat}}) }
+          end
+        end
+        {{original}}({{args.splat}})
+      end
+
+      def {{receiver}}{{name}}({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        if (%harness = ::Spectator::Harness.current?)
+          %args = ::Spectator::Mocks::GenericArguments.create({{args.splat}})
+          %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+          %harness.mocks.record_call(self, %call)
+          if (%stub = %harness.mocks.find_stub(self, %call))
+            return %stub.call!(%args) { {{original}}({{args.splat}}) { |*%ya| yield *%ya } }
+          end
+        end
+        {{original}}({{args.splat}}) do |*%yield_args|
+          yield *%yield_args
+        end
+      end
+    end
+  end
+end
diff --git a/src/spectator/mocks/type_registry.cr b/src/spectator/mocks/type_registry.cr
new file mode 100644
index 0000000..c3cb0b6
--- /dev/null
+++ b/src/spectator/mocks/type_registry.cr
@@ -0,0 +1,25 @@
+module Spectator::Mocks
+  module TypeRegistry
+    extend self
+
+    alias Key = Tuple(String, Symbol)
+
+    @@entries = {} of Key => Deque(MethodStub)
+
+    def add(type_name : String, method_name : Symbol, source : Source, args : Arguments) : Nil
+      key = {type_name, method_name}
+      list = if @@entries.has_key?(key)
+               @@entries[key]
+             else
+               @@entries[key] = Deque(MethodStub).new
+             end
+      list << NilMethodStub.new(method_name, source, args)
+    end
+
+    def exists?(type_name : String, call : GenericMethodCall(T, NT)) : Bool forall T, NT
+      key = {type_name, call.name}
+      list = @@entries.fetch(key) { return false }
+      list.any?(&.callable?(call))
+    end
+  end
+end
diff --git a/src/spectator/mocks/unexpected_message_error.cr b/src/spectator/mocks/unexpected_message_error.cr
new file mode 100644
index 0000000..9207846
--- /dev/null
+++ b/src/spectator/mocks/unexpected_message_error.cr
@@ -0,0 +1,4 @@
+module Spectator::Mocks
+  class UnexpectedMessageError < Exception
+  end
+end
diff --git a/src/spectator/mocks/value_method_stub.cr b/src/spectator/mocks/value_method_stub.cr
new file mode 100644
index 0000000..43bfcaa
--- /dev/null
+++ b/src/spectator/mocks/value_method_stub.cr
@@ -0,0 +1,14 @@
+require "./generic_arguments"
+require "./generic_method_stub"
+
+module Spectator::Mocks
+  class ValueMethodStub(ReturnType) < GenericMethodStub(ReturnType)
+    def initialize(name, source, @value : ReturnType, args = nil)
+      super(name, source, args)
+    end
+
+    def call(_args : GenericArguments(T, NT), &_original : -> RT) forall T, NT, RT
+      @value
+    end
+  end
+end
diff --git a/src/spectator/mocks/verifying_double.cr b/src/spectator/mocks/verifying_double.cr
new file mode 100644
index 0000000..3f451cb
--- /dev/null
+++ b/src/spectator/mocks/verifying_double.cr
@@ -0,0 +1,108 @@
+module Spectator::Mocks
+  abstract class VerifyingDouble(T)
+    def initialize(@null = false)
+    end
+
+    private macro stub(definition, &block)
+      {%
+        name = nil
+        params = nil
+        args = nil
+        body = nil
+        if definition.is_a?(Call) # stub foo { :bar }
+          named = false
+          name = definition.name.id
+          params = definition.args
+          args = params.map do |p|
+            n = p.is_a?(TypeDeclaration) ? p.var : p.id
+            r = named ? "#{n}: #{n}".id : n
+            named = true if n.starts_with?('*')
+            r
+          end
+          body = definition.block.is_a?(Nop) ? block : definition.block
+        elsif definition.is_a?(TypeDeclaration) # stub foo : Symbol
+          name = definition.var
+          params = [] of MacroId
+          args = [] of MacroId
+          body = block
+        else
+          raise "Unrecognized stub format"
+        end
+      %}
+
+      def {{name}}({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        %args = ::Spectator::Mocks::GenericArguments.create({{args.splat}})
+        %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+        ::Spectator::Harness.current.mocks.record_call(self, %call)
+
+        unless ::Spectator::Mocks::TypeRegistry.exists?(T.to_s, %call)
+          raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{name}} - #{T} does not respond to #{%call}")
+        end
+
+        if (%stub = ::Spectator::Harness.current.mocks.find_stub(self, %call))
+          %stub.call!(%args) { %method({{args.splat}}) }
+        else
+          %method({{args.splat}})
+        end
+      end
+
+      def {{name}}({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        %args = ::Spectator::Mocks::GenericArguments.create({{args.splat}})
+        %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+        ::Spectator::Harness.current.mocks.record_call(self, %call)
+
+        unless ::Spectator::Mocks::TypeRegistry.exists?(T.to_s, %call)
+          raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{name}} - #{T} does not respond to #{%call}")
+        end
+
+        if (%stub = ::Spectator::Harness.current.mocks.find_stub(self, %call))
+          %stub.call!(%args) { %method({{args.splat}}) { |*%ya| yield *%ya } }
+        else
+          %method({{args.splat}}) do |*%yield_args|
+            yield *%yield_args
+          end
+        end
+      end
+
+      def %method({{params.splat}}){% if definition.is_a?(TypeDeclaration) %} : {{definition.type}}{% end %}
+        {% if body && !body.is_a?(Nop) %}
+          {{body.body}}
+        {% else %}
+          %args = ::Spectator::Mocks::GenericArguments.create({{params.splat}})
+          %call = ::Spectator::Mocks::GenericMethodCall.new({{name.symbolize}}, %args)
+          unless ::Spectator::Harness.current.mocks.expected?(self, %call)
+            raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{name}}")
+          end
+
+          # This code shouldn't be reached, but makes the compiler happy to have a matching return type.
+          {% if definition.is_a?(TypeDeclaration) %}
+            %x = uninitialized {{definition.type}}
+          {% else %}
+            nil
+          {% end %}
+        {% end %}
+      end
+    end
+
+    macro method_missing(call)
+      args = ::Spectator::Mocks::GenericArguments.create({{call.args.splat}})
+      call = ::Spectator::Mocks::GenericMethodCall.new({{call.name.symbolize}}, args)
+      ::Spectator::Harness.current.mocks.record_call(self, call)
+
+      unless ::Spectator::Mocks::TypeRegistry.exists?(T.to_s, call)
+        raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{call.name}} - #{T} does not respond to #{call}")
+      end
+
+      return self if @null
+      return self if ::Spectator::Harness.current.mocks.expected?(self, call)
+
+      raise ::Spectator::Mocks::UnexpectedMessageError.new("#{self} received unexpected message {{call.name}}")
+    end
+
+    def to_s(io)
+      io << "Double("
+      io << T
+      io << ')'
+    end
+  end
+end
diff --git a/src/spectator/runnable_example.cr b/src/spectator/runnable_example.cr
index 9bbbbed..02086d0 100644
--- a/src/spectator/runnable_example.cr
+++ b/src/spectator/runnable_example.cr
@@ -20,7 +20,7 @@ module Spectator
         context.run_before_hooks(self)
         run_example(result)
         context.run_after_hooks(self)
-        run_deferred(result)
+        run_deferred(result) unless result.error
       end
     end
 
diff --git a/src/spectator/spec_builder.cr b/src/spectator/spec_builder.cr
index e8ec95e..9e199c2 100644
--- a/src/spectator/spec_builder.cr
+++ b/src/spectator/spec_builder.cr
@@ -96,6 +96,10 @@ module Spectator
       @@stack.current.add_post_condition(block)
     end
 
+    def add_default_stub(*args) : Nil
+      @@stack.current.add_default_stub(*args)
+    end
+
     # Builds the entire spec and returns it as a test suite.
     # This should be called only once after the entire spec has been defined.
     protected def build(filter : ExampleFilter) : TestSuite
diff --git a/src/spectator/spec_builder/example_group_builder.cr b/src/spectator/spec_builder/example_group_builder.cr
index f9ed5bf..950f99d 100644
--- a/src/spectator/spec_builder/example_group_builder.cr
+++ b/src/spectator/spec_builder/example_group_builder.cr
@@ -14,6 +14,7 @@ module Spectator::SpecBuilder
     @around_each_hooks = Deque(::SpectatorTest, Proc(Nil) ->).new
     @pre_conditions = Deque(TestMetaMethod).new
     @post_conditions = Deque(TestMetaMethod).new
+    @default_stubs = {} of String => Deque(Mocks::MethodStub)
 
     def add_child(child : Child)
       @children << child
@@ -47,6 +48,12 @@ module Spectator::SpecBuilder
       @post_conditions << hook
     end
 
+    def add_default_stub(type : T.class, stub : Mocks::MethodStub) forall T
+      key = type.name
+      @default_stubs[key] = Deque(Mocks::MethodStub).new unless @default_stubs.has_key?(key)
+      @default_stubs[key].unshift(stub)
+    end
+
     private def build_hooks
       ExampleHooks.new(
         @before_all_hooks.to_a,
diff --git a/src/spectator/spec_builder/nested_example_group_builder.cr b/src/spectator/spec_builder/nested_example_group_builder.cr
index dc08cff..6d7bb77 100644
--- a/src/spectator/spec_builder/nested_example_group_builder.cr
+++ b/src/spectator/spec_builder/nested_example_group_builder.cr
@@ -7,7 +7,7 @@ module Spectator::SpecBuilder
     end
 
     def build(parent_group)
-      context = TestContext.new(parent_group.context, build_hooks, build_conditions, parent_group.context.values)
+      context = TestContext.new(parent_group.context, build_hooks, build_conditions, parent_group.context.values, @default_stubs)
       NestedExampleGroup.new(@description, @source, parent_group, context).tap do |group|
         group.children = children.map do |child|
           child.build(group).as(ExampleComponent)
diff --git a/src/spectator/spec_builder/root_example_group_builder.cr b/src/spectator/spec_builder/root_example_group_builder.cr
index 02f4914..3dbbcf7 100644
--- a/src/spectator/spec_builder/root_example_group_builder.cr
+++ b/src/spectator/spec_builder/root_example_group_builder.cr
@@ -4,7 +4,7 @@ require "./example_group_builder"
 module Spectator::SpecBuilder
   class RootExampleGroupBuilder < ExampleGroupBuilder
     def build
-      context = TestContext.new(nil, build_hooks, build_conditions, TestValues.empty)
+      context = TestContext.new(nil, build_hooks, build_conditions, TestValues.empty, {} of String => Deque(Mocks::MethodStub))
       RootExampleGroup.new(context).tap do |group|
         group.children = children.map do |child|
           child.build(group).as(ExampleComponent)
diff --git a/src/spectator/spec_builder/sample_example_group_builder.cr b/src/spectator/spec_builder/sample_example_group_builder.cr
index eacd596..955ac7a 100644
--- a/src/spectator/spec_builder/sample_example_group_builder.cr
+++ b/src/spectator/spec_builder/sample_example_group_builder.cr
@@ -9,7 +9,7 @@ module Spectator::SpecBuilder
     def build(parent_group)
       values = parent_group.context.values
       collection = @collection_builder.call(values)
-      context = TestContext.new(parent_group.context, build_hooks, build_conditions, values)
+      context = TestContext.new(parent_group.context, build_hooks, build_conditions, values, @default_stubs)
       NestedExampleGroup.new(@description, @source, parent_group, context).tap do |group|
         group.children = collection.map do |element|
           build_sub_group(group, element).as(ExampleComponent)
@@ -19,7 +19,7 @@ module Spectator::SpecBuilder
 
     private def build_sub_group(parent_group, element)
       values = parent_group.context.values.add(@id, @description.to_s, element)
-      context = TestContext.new(parent_group.context, ExampleHooks.empty, ExampleConditions.empty, values)
+      context = TestContext.new(parent_group.context, ExampleHooks.empty, ExampleConditions.empty, values, {} of String => Deque(Mocks::MethodStub))
       NestedExampleGroup.new("#{@label} = #{element.inspect}", @source, parent_group, context).tap do |group|
         group.children = children.map do |child|
           child.build(group).as(ExampleComponent)
diff --git a/src/spectator/test_context.cr b/src/spectator/test_context.cr
index 8d16fee..bf2c612 100644
--- a/src/spectator/test_context.cr
+++ b/src/spectator/test_context.cr
@@ -3,9 +3,17 @@ require "./test_values"
 
 module Spectator
   class TestContext
+    getter! parent
+
     getter values
 
-    def initialize(@parent : TestContext?, @hooks : ExampleHooks, @conditions : ExampleConditions, @values : TestValues)
+    getter stubs : Hash(String, Deque(Mocks::MethodStub))
+
+    def initialize(@parent : TestContext?,
+                   @hooks : ExampleHooks,
+                   @conditions : ExampleConditions,
+                   @values : TestValues,
+                   @stubs : Hash(String, Deque(Mocks::MethodStub)))
       @before_all_hooks_run = false
       @after_all_hooks_run = false
     end
diff --git a/src/spectator/test_values.cr b/src/spectator/test_values.cr
index d07da71..8503ca8 100644
--- a/src/spectator/test_values.cr
+++ b/src/spectator/test_values.cr
@@ -18,8 +18,8 @@ module Spectator
     # Adds a new value by duplicating the current set and adding to it.
     # The new sample values with the additional value is returned.
     # The original set of sample values is not modified.
-    def add(id : Symbol, name : String, value : T) : TestValues forall T
-      wrapper = TypedValueWrapper(T).new(value)
+    def add(id : Symbol, name : String, value) : TestValues
+      wrapper = TypedValueWrapper.new(value)
       TestValues.new(@values.merge({
         id => Entry.new(name, wrapper),
       }))