Most workers in defined contribution retirement plans are inattentive portfolio managers: only a few engage in any trading at all, and only a tiny minority trades actively. Using a rich new dataset on 1.2 million workers in over 1,500 plans, we find that most 401(k) plan participants are characterized by profound inertia. Almost all participants (80%) initiate no trades, and an additional 11% makes only a single trade, in a two-year period. Even among traders, portfolio turnover rates are one-third the rate of professional money managers. Those who trade in their 401(k) plans are more affluent older men, with higher incomes and longer job tenure. They tend to use the internet for 401(k) account access, hold a larger number of investment options, and are more likely to hold active equity funds rather than index or lifecycle funds. Some plan features, including offering own-employer stock, also raise trading levels.