Research showing that high-quality preschool benefits children's early learning and later life outcomes has led to increased state engagement in public preschool. However, mixed results from evaluations of two programs--Tennessee's Voluntary Pre-K program and Head Start--have left many policymakers unsure about how to ensure productive investments. This brief and the report on which it is based present the most rigorous evidence on the effects of preschool and clarify how the findings from Tennessee and Head Start relate to the larger body of research showing that high-quality preschool enhances children's school readiness by supporting substantial early learning gains in comparison to children who do not experience preschool and can have lasting impacts far into children's later years of school and life. Therefore, the issue is not whether preschool "works," but how to design and implement programs that ensure public preschool investments consistently deliver on their promise. (author abstract)