diff --git a/evaluator/builtins.go b/evaluator/builtins.go
new file mode 100644
index 0000000..064c580
--- /dev/null
+++ b/evaluator/builtins.go
@@ -0,0 +1,20 @@
+package evaluator
+
+import "monkey/object"
+
+var builtins = map[string]*object.Builtin{
+	"len": &object.Builtin{
+		Fn: func(args ...object.Object) object.Object {
+			if len(args) != 1 {
+				return new_error("wrong number of arguments, got=%d, want=1", len(args))
+			}
+
+			switch arg := args[0].(type) {
+			case *object.String:
+				return &object.Integer{Value: int64(len(arg.Value))}
+			default:
+				return new_error("argument to `len` not supported, got %s", args[0].Type())
+			}
+		},
+	},
+}
diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index fef25ea..8a8d3c9 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -111,14 +111,19 @@ func eval_program(program *ast.Program, env *object.Environment) object.Object {
 }
 
 func apply_function(fn object.Object, args []object.Object) object.Object {
-	function, ok := fn.(*object.Function)
-	if !ok {
-		return new_error("not a function: %s", fn.Type())
+	switch fn := fn.(type) {
+	case *object.Function:
+		extended_env := extend_function_env(fn, args)
+		evaluated := Eval(fn.Body, extended_env)
+		return unwrap_return_value(evaluated)
+
+	case *object.Builtin:
+		return fn.Fn(args...)
+
+	default:
+		return new_error("not a funciton: %s", fn.Type())
 	}
 
-	extended_env := extend_function_env(function, args)
-	evaluated := Eval(function.Body, extended_env)
-	return unwrap_return_value(evaluated)
 }
 
 func extend_function_env(fn *object.Function, args []object.Object) *object.Environment {
@@ -151,11 +156,14 @@ func eval_expression(expressions []ast.Expression, env *object.Environment) []ob
 }
 
 func eval_identifier(node *ast.Identifier, env *object.Environment) object.Object {
-	val, ok := env.Get(node.Value)
-	if !ok {
-		return new_error("identifier not found: " + node.Value)
+	if val, ok := env.Get(node.Value); ok {
+		return val
 	}
-	return val
+	if builtin, ok := builtins[node.Value]; ok {
+		return builtin
+	}
+
+	return new_error("identifier not found: " + node.Value)
 }
 
 func eval_block_statement(block *ast.BlockStatement, env *object.Environment) object.Object {
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index 058ae42..abd5b01 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -270,6 +270,37 @@ func TestStringConcatenation(l_test *testing.T) {
 	}
 }
 
+func TestBuiltinFunction(l_test *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{`len("")`, 0},
+		{`len("four")`, 4},
+		{`len("hello world")`, 11},
+		{`len(1)`, "argument to `len` not supported, got INTEGER"},
+		{`len("one", "two")`, "wrong number of arguments, got=2, want=1"},
+	}
+
+	for _, tt := range tests {
+		evaluated := test_eval(tt.input)
+
+		switch expected := tt.expected.(type) {
+		case int:
+			test_integer_object(l_test, evaluated, int64(expected))
+		case string:
+			error_object, ok := evaluated.(*object.Error)
+			if !ok {
+				l_test.Errorf("object is not Error, got=%T (%+v)", evaluated, evaluated)
+				continue
+			}
+			if error_object.Message != expected {
+				l_test.Errorf("wrong error message, expected=%q, got=%q", expected, error_object.Message)
+			}
+		}
+	}
+}
+
 // Helpers
 func test_eval(input string) object.Object {
 	l_lexer := lexer.New(input)
diff --git a/object/object.go b/object/object.go
index b4091a4..319b141 100644
--- a/object/object.go
+++ b/object/object.go
@@ -17,6 +17,7 @@ const (
 	ERROR_OBJECT        = "ERROR"
 	FUNCTION_OBJECT     = "FUNCTION"
 	STRING_OBJECT       = "STRING"
+	BUILTIN_OBJ         = "BUILTIN"
 )
 
 type Object interface {
@@ -109,4 +110,14 @@ type String struct {
 }
 
 func (s *String) Type() ObjectType { return STRING_OBJECT }
-func (s *String) Inspect() string { return s.Value }
+func (s *String) Inspect() string  { return s.Value }
+
+// Built-in functions
+type BuiltinFunction func(args ...Object) Object
+
+type Builtin struct {
+	Fn BuiltinFunction
+}
+
+func (b *Builtin) Type() ObjectType { return BUILTIN_OBJ }
+func (b *Builtin) Inspect() string  { return "Builtin Function" }