We discuss the evidence on the effectiveness of preschool programs using results from three well-known intervention studies: the Chicago Child-Parent Centers, High/Scope Perry Preschool Program, and the Carolina Abecedarian Project. Results from cost-benefit analyses of other programs for younger and older children also are reported. Given that the Child-Parent Center Program is an established, large-scale preschool program for which a cost-benefit analysis has been recently completed, we focus on this program. We examine the longer-term effects in more detail and we investigate the robustness of estimates used in the cost-benefit analysis. Depending on the assumptions made, our results indicate that the benefit-cost ratio for the preschool program offered by the Child-Parent Centers ranges from $5.98-$10.15. We find strong evidence that the consistently positive economic returns of high-quality preschool programs exceed most other educational interventions, especially those that begin during the school-age years such as reduced class sizes in the elementary grades, grade retention, and youth job training.