Research shows that participation in a high-quality preschool can improve young children's readiness skills for elementary school, positively influencing behavioral, social-emotional, and cognitive outcomes. Specifically, for children who may be at risk for academic challenges in early elementary school, attending a high-quality preschool can improve test scores and attendance, and it can reduce placement in special education and grade-level retention. However, some preschool program evaluations document that initial benefits may not persist into early elementary school. Some early childhood experts assert that the effects of preschool may diminish if curricula and instructional strategies from preschool through grade 3 are not well aligned. A second explanation for why initial benefits of preschool may not persist is that children who make early gains in preschool may not have the opportunity to maintain their growth rate or learning trajectory because early elementary instruction may focus on students who are less prepared and have low-level skills. This literature review aimed to better understand the research behind these two theories and focused on preschool and K-3 alignment and differentiated instruction in kindergarten and first grade. The review of differ entiated instruction excluded studies that focused exclusively on low-achieving students because of the priority on differentiated instruction as a way to help sustain the gains children make in preschool. (author abstract)