This paper evaluates the long-run effects of Head Start using large-scale, restricted 2000-2018 Census-ACS data linked to the SSA’s Numident file, which contains exact date and county of birth. Using the county rollout of Head Start between 1965 and 1980 and age-eligibility cutoffs for school entry, we find that Head Start generated large increases in adult human capital and economic self-sufficiency, including a 0.65-year increase in schooling, a 2.7-percent increase in high-school completion, an 8.5-percent increase in college enrollment, and a 39-percent increase in college completion. These estimates imply sizable, long-term returns to public investments in large-scale preschool programs.