Do I have to contribute to my plan?

No, but you'd probably be foolish not to, because it's a great deal - though not nearly as great a deal as a traditional pension, in which your employer kicks in all the dough, bears all the investment risk and handles the administration of your account. Given that few employers offer traditional pensions anymore - they're much more costly than defined contribution plans - a 401(k) is usually the best game in town.