Children's experiences prior to kindergarten entry are correlated with degree of cognitive development and school readiness as measured by standardized assessments of cognitive and linguistic performance. Children from economically poor and undereducated families are at elevated risk for lack of school readiness due to less knowledge and skill. This article reviews evidence from randomized controlled trials (RCTs) that were designed to test the hypothesis that preschool education, with an emphasis on seven particular classes of experiences, could be efficacious in improving readiness for school and subsequent academic achievement in reading and mathematics. Results indicate that the cumulative developmental toll that is measured reliably in high-risk samples of children beginning in the second year of life can be substantially reduced through a high-quality preschool program. This positive effect has been replicated in nine additional trials using RCT methodology. Additionally, long-term follow-up of the original study participants indicates not only improved performance in reading and mathematics in elementary and secondary school but also a reduction in special education placement and grade retention, among other practical benefits. Results are discussed with respect to public policy recommendations and suggestions for future research.