diff --git a/spec/spectator/mocks/double_spec.cr b/spec/spectator/mocks/double_spec.cr
index 98f38b0..0ded983 100644
--- a/spec/spectator/mocks/double_spec.cr
+++ b/spec/spectator/mocks/double_spec.cr
@@ -1,7 +1,7 @@
 require "../../spec_helper"
 
 Spectator.describe Spectator::Double do
-  subject(dbl) { Spectator::Double.new("foobar", foo: 42, bar: "baz") }
+  subject(dbl) { Spectator::Double({foo: Int32, bar: String}).new("foobar", foo: 42, bar: "baz") }
 
   it "responds to defined messages" do
     aggregate_failures do
diff --git a/src/spectator/mocks/abstract_arguments.cr b/src/spectator/mocks/abstract_arguments.cr
new file mode 100644
index 0000000..dacc43f
--- /dev/null
+++ b/src/spectator/mocks/abstract_arguments.cr
@@ -0,0 +1,5 @@
+module Spectator
+  # Untyped arguments to a method call (message).
+  abstract class AbstractArguments
+  end
+end
diff --git a/src/spectator/mocks/abstract_response.cr b/src/spectator/mocks/abstract_response.cr
new file mode 100644
index 0000000..81c9659
--- /dev/null
+++ b/src/spectator/mocks/abstract_response.cr
@@ -0,0 +1,11 @@
+module Spectator
+  # Untyped response to a method call (message).
+  abstract class AbstractResponse
+    # Name of the method this response is for.
+    getter method : Symbol
+
+    # Creates the base of the response.
+    def initialize(@method : Symbol)
+    end
+  end
+end
diff --git a/src/spectator/mocks/arguments.cr b/src/spectator/mocks/arguments.cr
index ed89947..6b531cc 100644
--- a/src/spectator/mocks/arguments.cr
+++ b/src/spectator/mocks/arguments.cr
@@ -1,10 +1,12 @@
+require "./abstract_arguments"
+
 module Spectator
   # Arguments used in a method call.
   #
   # Can also be used to match arguments.
   # *T* must be a `Tuple` type representing the positional arguments.
   # *NT* must be a `NamedTuple` type representing the keyword arguments.
-  class Arguments(T, NT)
+  class Arguments(T, NT) < AbstractArguments
     # Positional arguments.
     getter args : T
 
@@ -20,6 +22,11 @@ module Spectator
       new(args, kwargs)
     end
 
+    # Constructs an instance of empty arguments.
+    macro empty
+      {{@type.name(generic_args: false)}}.capture
+    end
+
     # Constructs a string representation of the arguments.
     def to_s(io : IO) : Nil
       io << '('
diff --git a/src/spectator/mocks/double.cr b/src/spectator/mocks/double.cr
index e65b316..d087aff 100644
--- a/src/spectator/mocks/double.cr
+++ b/src/spectator/mocks/double.cr
@@ -1,3 +1,4 @@
+require "./abstract_response"
 require "./unexpected_message"
 
 module Spectator
@@ -5,10 +6,20 @@ module Spectator
   #
   # Handles all messages (method calls), but only responds to those configured.
   # Methods called that were not configured will raise `UnexpectedMessage`.
-  class Double(Messages)
+  # `NT` must be a type of `NamedTuple` that maps method names to their return types.
+  class Double(NT)
+    # Stores responses to messages (method calls).
+    @responses : Array(AbstractResponse)
+
     # Creates a double with pre-configures responses.
     # A *name* can be provided, otherwise it is considered an anonymous double.
-    def initialize(@name : String? = nil, **@messages : **Messages)
+    def initialize(@responses : Array(AbstractResponse), @name : String? = nil)
+    end
+
+    def initialize(@name : String? = nil, **methods : **NT)
+      @responses = methods.map do |method, value|
+        Response.new(method, value).as(AbstractResponse)
+      end
     end
 
     # Utility returning the double's name as a string.
@@ -37,7 +48,7 @@ module Spectator
             {% if meth.double_splat %}**{{meth.double_splat}}, {% end %}
             {% if meth.block_arg %}&{{meth.block_arg}}{% elsif meth.accepts_block? %}&{% end %}
           ){% if meth.return_type %} : {{meth.return_type}}{% end %}{% if !meth.free_vars.empty? %} forall {{meth.free_vars.splat}}{% end %}
-            \{% if type = Messages[{{meth.name.symbolize}}] %}
+            \{% if type = NT[{{meth.name.symbolize}}] %}
               {% if meth.return_type %}
                 \{% if type <= {{meth.return_type}} %}
                   # Return type appears to match configured type.
@@ -70,7 +81,10 @@ module Spectator
     # Handle all methods but only respond to configured messages.
     # Raises an `UnexpectedMessage` error for non-configures messages.
     macro method_missing(call)
-      \{% if Messages.keys.includes?({{call.name.symbolize}}.id) %}
+      \{% if NT.keys.includes?({{call.name.symbolize}}.id) %}
+        # Find a suitable response.
+        call = MethodCall.capture({{call.name.symbolize}}, {{call.args.splat}})
+        response = @responses.find &.===(call)
         # Return configured response.
         @messages[{{call.name.symbolize}}]
       \{% else %}
diff --git a/src/spectator/mocks/method_call.cr b/src/spectator/mocks/method_call.cr
new file mode 100644
index 0000000..eb13919
--- /dev/null
+++ b/src/spectator/mocks/method_call.cr
@@ -0,0 +1,18 @@
+require "./abstract_arguments"
+require "./arguments"
+
+module Spectator
+  class MethodCall
+    getter method : Symbol
+
+    getter arguments : AbstractArguments
+
+    def initialize(@method : Symbol, @arguments : Arguments = Arguments.empty)
+    end
+
+    def self.capture(method : Symbol, *args, **kwargs)
+      arguments = Arguments.new(args, kwargs)
+      new(method, arguments)
+    end
+  end
+end
diff --git a/src/spectator/mocks/response.cr b/src/spectator/mocks/response.cr
new file mode 100644
index 0000000..0558f32
--- /dev/null
+++ b/src/spectator/mocks/response.cr
@@ -0,0 +1,24 @@
+require "./abstract_arguments"
+require "./abstract_response"
+
+module Spectator
+  class Response(T) < AbstractResponse
+    # Return value.
+    getter value : T
+
+    # Arguments the method must have been called with to provide this response.
+    # Is nil when there's no constraint - only the method name must match.
+    getter constraint : AbstractArguments?
+
+    # Creates the response.
+    def initialize(@method : Symbol, @value : T, @constraint : Arguments? = nil)
+    end
+
+    def ===(call : MethodCall)
+      return false if method != call.method
+      return true unless constraint = @constraint
+
+      constraint === call.arguments
+    end
+  end
+end