This paper evaluates the long-run effects of Head Start using large-scale, restricted administrative data. 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 investments in means-tested, public preschool programs. (author abstract)