diff --git a/pkg/evaluator/evaluator.go b/pkg/evaluator/evaluator.go
index c2c06c8..8898dfd 100644
--- a/pkg/evaluator/evaluator.go
+++ b/pkg/evaluator/evaluator.go
@@ -59,6 +59,20 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		env.Set(node.Name.Value, val)
 	case *ast.Identifier:
 		return evalIdentifier(node, env)
+	case *ast.FunctionLiteral:
+		params := node.Parameters
+		body := node.Body
+		return &object.Function{Parameters: params, Body: body, Env: env}
+	case *ast.CallExpression:
+		fn := Eval(node.Function, env)
+		if isError(fn) {
+			return fn
+		}
+		args := evalExpressions(node.Arguments, env)
+		if len(args) == 1 && isError(args[0]) {
+			return args[0]
+		}
+		return applyFunction(fn, args)
 	}
 	return nil
 }
@@ -184,6 +198,46 @@ func evalIdentifier(exp *ast.Identifier, env *object.Environment) object.Object
 	return val
 }
 
+func evalExpressions(
+	exps []ast.Expression,
+	env *object.Environment,
+) []object.Object {
+	var res []object.Object
+	for _, exp := range exps {
+		ev := Eval(exp, env)
+		if isError(ev) {
+			return []object.Object{ev}
+		}
+		res = append(res, ev)
+	}
+	return res
+}
+
+func applyFunction(fnObj object.Object, args []object.Object) object.Object {
+	fn, ok := fnObj.(*object.Function)
+	if !ok {
+		return newError("not a function: %s", fn.Type())
+	}
+	env := extendFunctionEnv(fn, args)
+	ret := Eval(fn.Body, env)
+	return unwrapReturnValue(ret)
+}
+
+func extendFunctionEnv(fn *object.Function, args []object.Object) *object.Environment {
+	env := object.NewEnclosedEnvironment(fn.Env)
+	for pi, param := range fn.Parameters {
+		env.Set(param.Value, args[pi])
+	}
+	return env
+}
+
+func unwrapReturnValue(obj object.Object) object.Object {
+	if ret, ok := obj.(*object.ReturnValue); ok {
+		return ret.Value
+	}
+	return obj
+}
+
 func isTruthy(obj object.Object) bool {
 	switch obj {
 	case _TRUE:
diff --git a/pkg/evaluator/evaluator_test.go b/pkg/evaluator/evaluator_test.go
index ae953c5..d46828e 100644
--- a/pkg/evaluator/evaluator_test.go
+++ b/pkg/evaluator/evaluator_test.go
@@ -221,6 +221,49 @@ func TestLetStatements(t *testing.T) {
 	}
 }
 
+func TestFunctionObject(t *testing.T) {
+	input := "fn(x) { x + 2; };"
+
+	evaluated := testEval(input)
+	fn, ok := evaluated.(*object.Function)
+	if !ok {
+		t.Fatalf("object is not Function. got=%T (%+v)", evaluated, evaluated)
+	}
+
+	if len(fn.Parameters) != 1 {
+		t.Fatalf("function has wrong parameters. Parameters=%+v",
+			fn.Parameters)
+	}
+
+	if fn.Parameters[0].String() != "x" {
+		t.Fatalf("parameter is not 'x'. got=%q", fn.Parameters[0])
+	}
+
+	expectedBody := "(x + 2)"
+
+	if fn.Body.String() != expectedBody {
+		t.Fatalf("body is not %q. got=%q", expectedBody, fn.Body.String())
+	}
+}
+
+func TestFunctionApplication(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"let identity = fn(x) { x; }; identity(5);", 5},
+		{"let identity = fn(x) { return x; }; identity(5);", 5},
+		{"let double = fn(x) { x * 2; }; double(5);", 10},
+		{"let add = fn(x, y) { x + y; }; add(5, 5);", 10},
+		{"let add = fn(x, y) { x + y; }; add(5 + 5, add(5, 5));", 20},
+		{"fn(x) { x; }(5)", 5},
+	}
+
+	for _, tt := range tests {
+		testIntegerObject(t, testEval(tt.input), tt.expected)
+	}
+}
+
 func testNullObject(t *testing.T, obj object.Object) bool {
 	if obj != _NULL {
 		t.Errorf("object is not NULL. got=%T (%+v)", obj, obj)
diff --git a/pkg/object/environment.go b/pkg/object/environment.go
index 9a06f43..04ace5e 100644
--- a/pkg/object/environment.go
+++ b/pkg/object/environment.go
@@ -4,12 +4,23 @@ func NewEnvironment() *Environment {
 	return &Environment{store: map[string]Object{}}
 }
 
+func NewEnclosedEnvironment(outer *Environment) *Environment {
+	return &Environment{
+		store: map[string]Object{},
+		outer: outer,
+	}
+}
+
 type Environment struct {
 	store map[string]Object
+	outer *Environment
 }
 
 func (e *Environment) Get(name string) (Object, bool) {
 	obj, ok := e.store[name]
+	if !ok && e.outer != nil {
+		obj, ok = e.outer.Get(name)
+	}
 	return obj, ok
 }
 
diff --git a/pkg/object/object.go b/pkg/object/object.go
index f1446ad..bb88b69 100644
--- a/pkg/object/object.go
+++ b/pkg/object/object.go
@@ -1,6 +1,12 @@
 package object
 
-import "fmt"
+import (
+	"bytes"
+	"fmt"
+	"strings"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/ast"
+)
 
 type ObjectType string
 
@@ -10,6 +16,7 @@ const (
 	NULL_OBJ         = "NULL"
 	RETURN_VALUE_OBJ = "RETURN"
 	ERROR_OBJ        = "ERROR"
+	FUNCTION_OBJ     = "FUNCTION"
 )
 
 type Object interface {
@@ -69,3 +76,24 @@ func (e *Error) Type() ObjectType {
 func (e *Error) Inspect() string {
 	return "ERROR: " + e.Message
 }
+
+type Function struct {
+	Parameters []*ast.Identifier
+	Body       *ast.BlockStatement
+	Env        *Environment
+}
+
+func (f *Function) Type() ObjectType {
+	return FUNCTION_OBJ
+}
+func (f *Function) Inspect() string {
+	var out bytes.Buffer
+	params := []string{}
+	for _, p := range f.Parameters {
+		params = append(params, p.Value)
+	}
+	out.WriteString("fn")
+	out.WriteString("(" + strings.Join(params, ", ") + ")")
+	out.WriteString(" {\n" + f.Body.String() + "\n}")
+	return out.String()
+}