Executive SummaryThe future paths of 401(k) contributions and withdrawals, and the associated path of 401(k) asset values, will affect both the preparation of future retirees for their retirement years and the income tax revenues of federal and state governments. In this paper, we project the future growth of assets in self-directed personal retirement plans, such as 401(k) plans, at age 65 for cohorts attaining that age between now and 2040. We also project the ratio of 401(k) assets at 65 to prior earnings, and the ratio of aggregate 401(k) account balances to GDP. While there is substantial uncertainty in the future path of 401(k) balances, our projections of the future means for these balances suggest that cohorts that attain age 65 in future decades will have accumulated substantially more retirement saving (in real dollars) than current retirement-age cohorts. Our projections also highlight the drag that preretirement withdrawals and management fees place on asset accumulation.