diff --git a/monkey/README.md b/monkey/README.md
index a2a01a1..25ba00e 100644
--- a/monkey/README.md
+++ b/monkey/README.md
@@ -109,6 +109,9 @@ is represented as:
 * `REPL` - `Read-Eval-Print Loop`
 * sometimes - 'console', 'interactive mode', etc.
 
+### built-in functions
+* design choice - should they be evalueated in top-level environment or in their own environment? (`object.Environment`)
+
 ### compiler
 * goal is turning `source code` into `bytecode` and then use VM to execute it
 
diff --git a/monkey/interpreter/evaluator/builtins.go b/monkey/interpreter/evaluator/builtins.go
new file mode 100644
index 0000000..3c5255c
--- /dev/null
+++ b/monkey/interpreter/evaluator/builtins.go
@@ -0,0 +1,22 @@
+package evaluator
+
+import (
+	"monkey/interpreter/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/monkey/interpreter/evaluator/evaluator.go b/monkey/interpreter/evaluator/evaluator.go
index e6b80ad..08f4107 100644
--- a/monkey/interpreter/evaluator/evaluator.go
+++ b/monkey/interpreter/evaluator/evaluator.go
@@ -96,14 +96,19 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 }
 
 func applyFunction(fn object.Object, args []object.Object) object.Object {
-	function, ok := fn.(*object.Function)
-	if !ok {
+	switch fn := fn.(type) {
+
+	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())
 	}
-
-	extendedEnv := extendFunctionEnv(function, args)
-	evaluated := Eval(function.Body, extendedEnv)
-	return unwrapReturnValue(evaluated)
 }
 
 func extendFunctionEnv(
@@ -178,13 +183,19 @@ func evalBlockStatement(block *ast.BlockStatement, env *object.Environment) obje
 	return result
 }
 
-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)
+func evalIdentifier(
+	node *ast.Identifier,
+	env *object.Environment,
+) object.Object {
+	if val, ok := env.Get(node.Value); ok {
+		return val
+	}
+
+	if builtins, ok := builtins[node.Value]; ok {
+		return builtins
 	}
 
-	return val
+	return newError("identifier not found: " + node.Value)
 }
 
 func nativeBooltoBooleanObject(input bool) *object.Boolean {
@@ -237,6 +248,8 @@ func evalInfixExpression(
 	// so we can compare them directly (checking for identity)
 	// this assumption does not hold for other types, because creating integer object allocates new memory
 	// so we need to compare their values (otherwise 5 == 5 would yield false)
+	case left.Type() == object.STRING_OBJ && right.Type() == object.STRING_OBJ:
+		return evalStringInfixExpression(operator, left, right)
 	case operator == "==":
 		return nativeBooltoBooleanObject(left == right)
 	case operator == "!=":
@@ -277,6 +290,19 @@ func evalIntegerInfixExpression(
 	}
 }
 
+func evalStringInfixExpression(operator string, left, right object.Object) object.Object {
+	leftVal := left.(*object.String).Value
+	rightVal := right.(*object.String).Value
+
+	switch operator {
+	case "+":
+		return &object.String{Value: leftVal + rightVal}
+	// TODO: maybe add more string operations (like comparison)
+	default:
+		return newError("unknown operator: %s %s %s", left.Type(), operator, right.Type())
+	}
+}
+
 func evalIfExpression(ie *ast.IfExpression, env *object.Environment) object.Object {
 	condition := Eval(ie.Condition, env)
 	if isError(condition) {
diff --git a/monkey/interpreter/evaluator/evaluator_test.go b/monkey/interpreter/evaluator/evaluator_test.go
index 1a49656..d26d506 100644
--- a/monkey/interpreter/evaluator/evaluator_test.go
+++ b/monkey/interpreter/evaluator/evaluator_test.go
@@ -184,6 +184,10 @@ func TestErrorHandling(t *testing.T) {
 			"foobar",
 			"identifier not found: foobar",
 		},
+		{
+			`"Hello" - "World"`,
+			"unknown operator: STRING - STRING",
+		},
 		{
 			`
 	if (10 > 1) {
@@ -273,6 +277,49 @@ func TestClosures(t *testing.T) {
 	testIntegerObject(t, testEval(input), 4)
 }
 
+func TestStringConcatenation(t *testing.T) {
+	input := `"Hello" + " " + "World!"`
+	evaluated := testEval(input)
+	str, ok := evaluated.(*object.String)
+	if !ok {
+		t.Fatalf("object is not String. got=%T (%+v)", evaluated, evaluated)
+	}
+	if str.Value != "Hello World!" {
+		t.Errorf("String has wrong value. got=%q", str.Value)
+	}
+}
+
+func TestBuiltinFunctions(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/monkey/interpreter/object/object.go b/monkey/interpreter/object/object.go
index c9215f6..a54a707 100644
--- a/monkey/interpreter/object/object.go
+++ b/monkey/interpreter/object/object.go
@@ -9,6 +9,8 @@ import (
 
 type ObjectType string
 
+type BuiltinFunction func(args ...Object) Object
+
 const (
 	INTEGER_OBJ      = "INTEGER"
 	BOOLEAN_OBJ      = "BOOLEAN"
@@ -17,6 +19,7 @@ const (
 	ERROR_OBJ        = "ERROR"
 	FUNCTION_OBJ     = "FUNCTION"
 	STRING_OBJ       = "STRING"
+	BUILTIN_OBJ      = "BUILTIN"
 )
 
 type Object interface {
@@ -88,3 +91,10 @@ func (f *Function) Inspect() string {
 
 	return out.String()
 }
+
+type Builtin struct {
+	Fn BuiltinFunction
+}
+
+func (b *Builtin) Inspect() string  { return "builtin function" }
+func (b *Builtin) Type() ObjectType { return BUILTIN_OBJ }