Cincinnati, Ohio, is one of a growing number of U.S. cities seeking to expand access to and raise the quality of preschool programs, especially for the city's most vulnerable children. In a companion report, we document the current preschool landscape in Cincinnati and consider design options for allowing more low-income children to enroll in high-quality programs. In this report, we aim to compile the most-reliable research evidence concerning whether high-quality preschool programs benefit participating children and whether the economic value of outcomes experienced by participating children exceeds the economic costs to implement the program. Our review draws on evidence from full-scale preschool programs implemented at the national, state, and local levels. (author abstract)