This chapter reviews the academic literature on defined contribution retirement plan design and participant behavior. While adoption of automatic enrollment has significantly increased participation rates, recent studies find the long-run effects on savings are smaller than the short-run effects, with some savings financed via debt. I also review efforts to expand access to employer-based retirement savings and liquid savings, the pros and cons of target date funds as default investment options, potential conflicts of interest in plan design, and potential benefits of customized defaults. I conclude by discussing how SECURE 2.0 may impact US workers and highlighting topics for future research.