diff --git a/src/chunk.zig b/src/chunk.zig
index 289ef33..76d4bb9 100644
--- a/src/chunk.zig
+++ b/src/chunk.zig
@@ -28,6 +28,9 @@ const AllOpcodes = struct {
 
     pub Print: u8 = 15,
     pub Pop: u8 = 16,
+
+    pub DefineGlobal: u8 = 17,
+    pub DefineGlobalLong: u8 = 18,
 };
 
 pub const OpCode = AllOpcodes{};
@@ -78,6 +81,16 @@ fn constantLongInstruction(
     return offset + 4;
 }
 
+pub const ConstantIndexTag = enum {
+    Small,
+    Long,
+};
+
+pub const ConstantIndex = union(ConstantIndexTag) {
+    Small: u8,
+    Long: [3]u8,
+};
+
 pub const Chunk = struct {
     count: usize,
     lines: []usize,
@@ -119,13 +132,20 @@ pub const Chunk = struct {
         return self.constants.count - 1;
     }
 
-    pub fn writeConstant(self: *Chunk, val: value.Value, line: usize) !void {
+    pub fn writeConstant(
+        self: *Chunk,
+        val: value.Value,
+        line: usize,
+    ) !ConstantIndex {
         try self.constants.write(val);
         var constant_idx = self.constants.count - 1;
 
         if (constant_idx < 256) {
             try self.write(OpCode.Constant, line);
-            try self.write(@intCast(u8, constant_idx), line);
+
+            var idx_small = @intCast(u8, constant_idx);
+            try self.write(idx_small, line);
+            return ConstantIndex{ .Small = idx_small };
         } else {
             var idx_u24: u24 = @intCast(u24, constant_idx);
 
@@ -139,6 +159,7 @@ pub const Chunk = struct {
             try self.write(v3, line);
             try self.write(v2, line);
             try self.write(v1, line);
+            return ConstantIndex{ .Long = []u8{ v3, v2, v1 } };
         }
     }
 
@@ -196,6 +217,10 @@ pub const Chunk = struct {
             return try simpleInstruction(stdout, "OP_PRINT", index);
         } else if (instruction == OpCode.Pop) {
             return try simpleInstruction(stdout, "OP_POP", index);
+        } else if (instruction == OpCode.DefineGlobal) {
+            return try simpleInstruction(stdout, "OP_DEFGLOBAL", index);
+        } else if (instruction == OpCode.DefineGlobalLong) {
+            return try simpleInstruction(stdout, "OP_DEFGLOBAL_LONG", index);
         } else {
             try stdout.print("Unknown opcode: {}\n", instruction);
             return index + 1;
diff --git a/src/compiler.zig b/src/compiler.zig
index 71750da..1b44624 100644
--- a/src/compiler.zig
+++ b/src/compiler.zig
@@ -215,7 +215,7 @@ pub const Compiler = struct {
     }
 
     fn emitConstant(self: *Compiler, value: Value) !void {
-        try self.currentChunk().writeConstant(
+        _ = try self.currentChunk().writeConstant(
             value,
             self.parser.previous.line,
         );
@@ -352,8 +352,54 @@ pub const Compiler = struct {
         }
     }
 
+    /// Write an identifier constant to the bytecode.
+    fn identifierConstant(
+        self: *Compiler,
+        token: *Token,
+    ) !chunks.ConstantIndex {
+        return try self.currentChunk().writeConstant(values.ObjVal(try objects.copyString(
+            self.vmach,
+            token.lexeme,
+        )), token.line);
+    }
+
+    fn parseVariable(self: *Compiler, msg: []const u8) !chunks.ConstantIndex {
+        try self.consume(.IDENTIFIER, msg);
+        return try self.identifierConstant(&self.parser.previous);
+    }
+
+    fn defineVariable(self: *Compiler, global: chunks.ConstantIndex) !void {
+        switch (global) {
+            .Small => |val| try self.emitBytes(chunks.OpCode.DefineGlobal, val),
+            .Long => |val| blk: {
+                try self.emitByte(chunks.OpCode.DefineGlobalLong);
+                try self.emitByte(val[0]);
+                try self.emitByte(val[1]);
+                try self.emitByte(val[2]);
+            },
+            else => unreachable,
+        }
+    }
+
+    fn varDecl(self: *Compiler) !void {
+        var global = try self.parseVariable("Expect variable name.");
+
+        if (try self.match(.EQUAL)) {
+            try self.expression();
+        } else {
+            try self.emitByte(chunks.OpCode.Nil);
+        }
+
+        try self.consume(.SEMICOLON, "Expect ';' after variable declaration.");
+        try self.defineVariable(global);
+    }
+
     fn declaration(self: *Compiler) !void {
-        try self.statement();
+        if (try self.match(.VAR)) {
+            try self.varDecl();
+        } else {
+            try self.statement();
+        }
         if (self.parser.panicMode) try self.synchronize();
     }
 
diff --git a/src/vm.zig b/src/vm.zig
index 98211c6..0b6e3fb 100644
--- a/src/vm.zig
+++ b/src/vm.zig
@@ -36,6 +36,8 @@ fn valuesEqual(a: value.Value, b: value.Value) bool {
     }
 }
 
+pub const ValueMap = std.AutoHashMap([]const u8, values.Value);
+
 pub const VM = struct {
     chk: *Chunk = undefined,
     src: []const u8,
@@ -49,6 +51,7 @@ pub const VM = struct {
     pub allocator: *std.mem.Allocator,
 
     objs: ?*objects.Object = null,
+    globals: ValueMap,
 
     fn resetStack(self: *VM) void {
         self.stackTop = 0;
@@ -67,6 +70,8 @@ pub const VM = struct {
             .stdout = stdout,
             .debug_flag = debug_flag,
             .allocator = allocator,
+
+            .globals = ValueMap.init(allocator),
         };
 
         self.resetStack();
@@ -219,6 +224,11 @@ pub const VM = struct {
         self.resetStack();
     }
 
+    fn defGlobal(self: *VM, name: []const u8) !void {
+        _ = try self.globals.put(name, self.peek(0));
+        _ = self.pop();
+    }
+
     fn run(self: *VM) !void {
         while (true) {
             if (self.debug_flag) {
@@ -260,6 +270,18 @@ pub const VM = struct {
                     break :blk;
                 },
 
+                // extracting the name is different depending of the
+                // op code since one just uses a single byte, the other
+                // uses three bytes since its a u24.
+                chunk.OpCode.DefineGlobal => blk: {
+                    try self.defGlobal(self.readConst().as.Object.value.String);
+                    break :blk;
+                },
+                chunk.OpCode.DefineGlobalLong => blk: {
+                    try self.defGlobal(self.readConstLong().as.Object.value.String);
+                    break :blk;
+                },
+
                 chunk.OpCode.Equal => blk: {
                     var a = self.pop();
                     var b = self.pop();