This chapter describes the goals and evolution of the Head Start program since its inception, and argues that despite fifty years of research on the program considerable debate remains about whether the program generates long-term improvements in the life chances of low-income children. As the research designs and data available to study Head Start have improved, we have seen accumulating evidence that for children participating in Head Start several decades ago, the program has had important long-term impacts on outcomes like schooling attainment and earnings despite fade-out of the program's initial impacts on achievement test scores. A similar pattern has been observed for other early childhood interventions as well. For more recent cohorts of Head Start children, for whom only short-run impacts can be measured, we see similar signs of short-term test score impacts that then dissipate. One common hypothesis is that Head Start and other early interventions have impacts on nonacademic or "social-cognitive" skills that mediate long-term improvements in adult outcomes. Unfortunately, too little is currently known about how this process works to determine whether the children participating in Head Start today will experience the same long-term benefits experienced by previous program participants. Current claims that Head Start does not work may be too pessimistic, or at the very least premature. (author abstract)