We used longitudinal data from a birth cohort study, the Fragile Families and Child Wellbeing Study, to investigate the links between Head Start and school readiness in a large and diverse sample of urban children at age 5 (N = 2,803; 18 cities). We found that Head Start attendance was associated with enhanced cognitive ability and social competence and reduced attention problems but not reduced internalizing or externalizing behavior problems. These findings were robust to model specifications (including models with city-fixed effects and propensity-scoring matching). Furthermore, the effects of Head Start varied by the reference group. Head Start was associated with improved cognitive development when compared with parental care or other nonparental care, as well as improved social competence (compared with parental care) and reduced attention problems (compared with other nonparental care). In contrast, compared with attendance at pre-kindergarten or other center-based care, Head Start attendance was not associated with cognitive gains but with improved social competence and reduced attention and externalizing behavior problems (compared with attendance at other center-based care). These associations were not moderated by child gender or race/ethnicity. (Contains 18 footnotes, 8 tables, and 1 figure.)