This is the first study to examine both how well plan administrators select funds for 401(k) plans and how participants react to plan administrator decisions. We find that, on average, administrators select funds that outperform randomly selected funds of the same type although they do not outperform index funds of the same type. When administrators change offerings, they choose funds that did well in the past, but, after the change, added funds do no better than dropped funds. Plan participants in aggregate change their allocation decisions in a way that accentuates the changes in allocation caused by returns. The change in allocation due to the investment of new money and interfund transfers is about the same size, and in the same direction, as the change due to returns. Participant allocations in aggregate do no better than naïve allocation rules, such as equal investment in each offering.