diff --git a/evaluator/builtins.go b/evaluator/builtins.go
new file mode 100644
index 0000000..989e7c9
--- /dev/null
+++ b/evaluator/builtins.go
@@ -0,0 +1,21 @@
+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 newError("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 newError("argument to `len` not supported, got %s", args[0].Type())
+
+			}
+		},
+	},
+}
diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index 6969853..b2f9213 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -264,11 +264,15 @@ func newError(format string, a ...interface{}) *object.Error {
 }
 
 func evalIdentifier(node *ast.Identifier, env *object.Environment) object.Object {
-	val, ok := env.Get(node.Value)
-	if !ok {
-		return newError("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 newError("identifier not found: " + node.Value)
 }
 
 func evalExpressions(exps []ast.Expression, env *object.Environment) []object.Object {
@@ -286,14 +290,20 @@ func evalExpressions(exps []ast.Expression, env *object.Environment) []object.Ob
 }
 
 func applyFunction(fn object.Object, args []object.Object) object.Object {
-	function, ok := fn.(*object.Function)
-	if !ok {
-		newError("not a function: %s", fn.Type())
-	}
+	switch fn := fn.(type) {
 
-	extendedEnv := extendFunctionEnv(function, args)
-	evaluated := Eval(function.Body, extendedEnv)
-	return unwrapReturnValue(evaluated)
+	case *object.Function:
+		extendedEnv := extendFunctionEnv(fn, args)
+		evaluated := Eval(fn.Body, extendedEnv)
+		return unwrapReturnValue(evaluated)
+
+	case *object.Builtin:
+		return fn.Fn(args...)
+
+	default:
+		return newError("not a function: %s", fn.Type())
+
+	}
 }
 
 func extendFunctionEnv(fn *object.Function, args []object.Object) *object.Environment {
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index dffabef..160258b 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -326,6 +326,37 @@ func TestStringConcatenation(t *testing.T) {
 	}
 }
 
+func TestBuiltinFunction(t *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 := testEval(tt.input)
+
+		switch expected := tt.expected.(type) {
+		case int:
+			testIntegerObject(t, evaluated, int64(expected))
+		case string:
+			errObj, ok := evaluated.(*object.Error)
+			if !ok {
+				t.Errorf("object is not Error. got=%T (%+v)", evaluated, evaluated)
+				continue
+			}
+			if errObj.Message != expected {
+				t.Errorf("wrong error message. expected=%q, got=%q", expected, errObj.Message)
+			}
+		}
+	}
+}
+
 func testEval(input string) object.Object {
 	l := lexer.New(input)
 	p := parser.New(l)
diff --git a/object/object.go b/object/object.go
index 7d2db1d..6b1aa65 100644
--- a/object/object.go
+++ b/object/object.go
@@ -17,6 +17,7 @@ const (
 	ERROR_OBJ        = "ERROR"
 	FUNCTION_OBJ     = "FUNCTION"
 	STRING_OBJ       = "STRING"
+	BUILTIN_OBJ      = "BUILTIN"
 )
 
 type Object interface {
@@ -119,3 +120,16 @@ func (s *String) Type() ObjectType {
 func (s *String) Inspect() string {
 	return s.Value
 }
+
+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"
+}