This paper estimates the effect of Head Start on health, education, and labor market outcomes observed through age 48. I combine outcome data from the NLSY79 with archival records on early Head Start funding levels, and for identification exploit differences across counties in the introduction timing and size of local Head Start programs. This allows me to compare the long-term outcomes of children who were too old for Head Start when the program was introduced in their county with the outcomes of children who were sufficiently young to be eligible. I find that individuals from counties that had an average sized program when they were in Head Start's target age range experienced a $2,199 increase in annual adult earnings, completed .125 additional years of education, were 4.6 percentage points less likely to have a health limitation at age 40, and overall experienced a .081 standard deviation improvement in a summary index of these and other outcome measures. Funding levels at ages outside of Head Start's target range are not significantly correlated with long-term outcomes. Estimated treatment effects are largest among blacks, the children of lower-education parents, and children exposed to better funded Head Start programs, heterogeneity that is consistent with a causal program impact. (author abstract)