A growing body of research suggests that the first few years of life are a particularly promising time to intervene in the lives of low-income children, although the long-term effects on children of the U.S. government's primary early childhood program - Head Start - remains the topic of debate. In this article we review what is known about Head Start and argue that the program is likely to generate benefits to participants and society as a whole that are large enough to justify the program's costs. Although in principle there could be more beneficial ways of deploying Head Start resources, the benefits of such changes remain uncertain and there is some downside risk. (author abstract)