We use data from the Health and Retirement Study linked to administrative data on earnings and benefits to determine why some workers end up with low Social Security benefits in retirement. Several characteristics are associated with family benefits of less than poverty. Racial disparities are pronounced. Women's risk is marked, especially for unmarried women, with caregiving an important contributor to low-benefit risk. Less-educated workers are also vulnerable, sometimes even when they have worked long careers. Workers with health problems and disabled workers-especially those disabled early in the career-are comparatively likely to have family benefits of less than poverty.