diff --git a/pkg/evaluator/builtins.go b/pkg/evaluator/builtins.go
index 659524a..d85f02f 100644
--- a/pkg/evaluator/builtins.go
+++ b/pkg/evaluator/builtins.go
@@ -1,6 +1,10 @@
 package evaluator
 
-import "code.jmug.me/jmug/interpreter-in-go/pkg/object"
+import (
+	"os"
+
+	"code.jmug.me/jmug/interpreter-in-go/pkg/object"
+)
 
 var builtins = map[string]*object.Builtin{
 	"len": {
@@ -13,10 +17,96 @@ var builtins = map[string]*object.Builtin{
 			switch arg := args[0].(type) {
 			case *object.String:
 				return &object.Integer{Value: int64(len(arg.Value))}
+			case *object.Array:
+				return &object.Integer{Value: int64(len(arg.Elements))}
 			default:
 				return newError("argument to `len` not supported, got %s",
 					args[0].Type())
 			}
 		},
 	},
+	"first": {
+		Fn: func(args ...object.Object) object.Object {
+			if len(args) != 1 {
+				return newError("wrong number of arguments. got=%d, want=1",
+					len(args))
+			}
+			if args[0].Type() != object.ARRAY_OBJ {
+				return newError("argument to `first` must be ARRAY, got %s",
+					args[0].Type())
+			}
+
+			arr := args[0].(*object.Array)
+			if len(arr.Elements) > 0 {
+				return arr.Elements[0]
+			}
+
+			return _NULL
+		},
+	},
+	"last": {
+		Fn: func(args ...object.Object) object.Object {
+			if len(args) != 1 {
+				return newError("wrong number of arguments. got=%d, want=1",
+					len(args))
+			}
+			if args[0].Type() != object.ARRAY_OBJ {
+				return newError("argument to `last` must be ARRAY, got %s",
+					args[0].Type())
+			}
+
+			arr := args[0].(*object.Array)
+			if len(arr.Elements) > 0 {
+				return arr.Elements[len(arr.Elements)-1]
+			}
+			return _NULL
+		},
+	},
+	"rest": {
+		Fn: func(args ...object.Object) object.Object {
+			if len(args) != 1 {
+				return newError("wrong number of arguments. got=%d, want=1",
+					len(args))
+			}
+			if args[0].Type() != object.ARRAY_OBJ {
+				return newError("argument to `rest` must be ARRAY, got %s",
+					args[0].Type())
+			}
+			arr := args[0].(*object.Array).Elements
+			arrLen := len(arr)
+			if arrLen > 0 {
+				newArr := make([]object.Object, arrLen-1)
+				copy(newArr, arr[1:])
+				return &object.Array{Elements: newArr}
+			}
+			return _NULL
+		},
+	},
+	"push": {
+		Fn: func(args ...object.Object) object.Object {
+			if len(args) != 2 {
+				return newError("wrong number of arguments. got=%d, want=2",
+					len(args))
+			}
+			if args[0].Type() != object.ARRAY_OBJ {
+				return newError("argument to `push` must be ARRAY, got %s",
+					args[0].Type())
+			}
+			arr := args[0].(*object.Array).Elements
+			arrLen := len(arr)
+			newArr := make([]object.Object, arrLen+1)
+			copy(newArr, arr)
+			newArr[arrLen] = args[1]
+			return &object.Array{Elements: newArr}
+		},
+	},
+	"exit": {
+		Fn: func(args ...object.Object) object.Object {
+			if len(args) != 0 {
+				return newError("exit takes no arguments...")
+			}
+			os.Exit(0)
+			return nil // Make the compiler happy.
+		},
+	},
 }
diff --git a/pkg/evaluator/evaluator.go b/pkg/evaluator/evaluator.go
index f910a3e..82edc36 100644
--- a/pkg/evaluator/evaluator.go
+++ b/pkg/evaluator/evaluator.go
@@ -77,10 +77,20 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		return applyFunction(fn, args)
 	case *ast.ArrayLiteral:
 		els := evalExpressions(node.Elements, env)
-		if len(els) == 1 && isError(els[1]) {
+		if len(els) == 1 && isError(els[0]) {
 			return els[0]
 		}
 		return &object.Array{Elements: els}
+	case *ast.IndexExpression:
+		left := Eval(node.Left, env)
+		if isError(left) {
+			return left
+		}
+		index := Eval(node.Index, env)
+		if isError(index) {
+			return index
+		}
+		return evalIndexExpression(left, index)
 	}
 	return nil
 }
@@ -237,6 +247,24 @@ func evalExpressions(
 	return res
 }
 
+func evalIndexExpression(left, index object.Object) object.Object {
+	switch {
+	case left.Type() == object.ARRAY_OBJ && index.Type() == object.INTEGER_OBJ:
+		return evalArrayIndexExpression(left, index)
+	default:
+		return newError("index operator not supported: %s", left.Type())
+	}
+}
+
+func evalArrayIndexExpression(arrayObj, indexObj object.Object) object.Object {
+	array := arrayObj.(*object.Array).Elements
+	index := indexObj.(*object.Integer).Value
+	if index < 0 || index >= int64(len(array)) {
+		return _NULL
+	}
+	return array[index]
+}
+
 func applyFunction(fnObj object.Object, args []object.Object) object.Object {
 	switch fn := fnObj.(type) {
 	case *object.Function:
diff --git a/pkg/evaluator/evaluator_test.go b/pkg/evaluator/evaluator_test.go
index 51c311c..938c47f 100644
--- a/pkg/evaluator/evaluator_test.go
+++ b/pkg/evaluator/evaluator_test.go
@@ -348,6 +348,64 @@ func TestArrayLiterals(t *testing.T) {
 	testIntegerObject(t, result.Elements[2], 6)
 }
 
+func TestArrayIndexExpressions(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{
+			"[1, 2, 3][0]",
+			1,
+		},
+		{
+			"[1, 2, 3][1]",
+			2,
+		},
+		{
+			"[1, 2, 3][2]",
+			3,
+		},
+		{
+			"let i = 0; [1][i];",
+			1,
+		},
+		{
+			"[1, 2, 3][1 + 1];",
+			3,
+		},
+		{
+			"let myArray = [1, 2, 3]; myArray[2];",
+			3,
+		},
+		{
+			"let myArray = [1, 2, 3]; myArray[0] + myArray[1] + myArray[2];",
+			6,
+		},
+		{
+			"let myArray = [1, 2, 3]; let i = myArray[0]; myArray[i]",
+			2,
+		},
+		{
+			"[1, 2, 3][3]",
+			nil,
+		},
+		{
+			"[1, 2, 3][-1]",
+			nil,
+		},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		integer, ok := tt.expected.(int)
+		if ok {
+			testIntegerObject(t, evaluated, int64(integer))
+		} else {
+			testNullObject(t, evaluated)
+		}
+	}
+}
+
 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/token/token.go b/pkg/token/token.go
index 3864c3b..0ccca72 100644
--- a/pkg/token/token.go
+++ b/pkg/token/token.go
@@ -26,6 +26,7 @@ const (
 	// Delimiters
 	COMMA     = ","
 	SEMICOLON = ";"
+	COLON     = ":"
 
 	LPAREN   = "("
 	RPAREN   = ")"