Social, economic, demographic, and public policy shifts have made millennial retirement security a pressing concern. Using DYNASIM4, the Urban Institute’s dynamic microsimulation model, we project that per capita family income at age 70 will increase over time. However, the share of adults with age-70 income insufficient to cover basic needs and to maintain preretirement living standards is projected to grow. We estimate that 38 percent of early millennials born in the 1980s will have inadequate age-70 income, compared with 28 percent of preboomers (born between 1937 and 1945) and 30 percent of late boomers (born between 1955 and 1964). Retirement security is projected to be especially precarious for early millennials of color, those with little education and limited lifetime earnings, and those who are not married. The possibility that Social Security’s long-term financing gap could lead to future benefit cuts further clouds millennials’ retirement outlook.