The review distills lessons for policymakers by combining a detailed description of preschool program evaluations and research methodologies (see Appendix B and C) with an accessible discussion of their findings. Specifically, it unpacks the challenges that pervade the discussion around preschool, including inconsistencies in study methods, and hypothesizes about other factors that may explain the findings of the preschool literature, such as the quality of preschool programs and subsequent elementary school experiences. The ultimate goal of this review is to equip policymakers and others with the tools to assess the weight of accumulated evidence in light of these challenges and draw conclusions about the case for investing in preschool. (author abstract)