PROJECT SUMMARY Preschool or kindergarten represents the start of children’s educational careers. These early educational opportunities are viewed as critical investments because they offer a great deal of promise for improving children’s school success (Phillips et al., 2017). With that said, there remain questions about how these early experiences shape social and behavioral development (Huston et al., 2015). To date, much of the research on these early years has focused on academic outcomes and examines only preschool or kindergarten. This is a significant oversight given that schools are more than academic institutions (Rimm-Kaufman et al., 2002) and children’s preschool experiences may shape their adaptation to kindergarten (Vitiello et al., 2020). Given the key role early social and behavioral skills have in shaping later health and well-being (Jones et al., 2015), understanding the ways in which they are shaped by classroom environments is a key public health question. Given the above, understanding how malleable aspects of the preschool and kindergarten classroom, and the alignment across them (or lack thereof), contribute to children’s development is key to ensuring that investments in preschool are maximized. This is particularly important with regards to children’s social behavioral development given findings that suggest the long-term benefits of early educational experiences manifest through changes in these domains (e.g., Gray-Lobe et al., 2021). Despite this emerging pattern of findings, our knowledge of the social behavioral benefits of early education has been greatly limited. The current proposal seeks to further our understanding regarding the key aspects of children’s preschool and kindergarten classroom experiences and their predictive associations with children’s social and behavioral development. We plan to leverage three large, diverse, and longitudinal datasets of preschoolers and kindergarteners that collectively allow for a more comprehensive assessment of children’s educational experiences. We will use these data to: 1) examine what aspects of the kindergarten classroom context shape children’s social and behavioral development, and how these patterns vary for children who did or did not attend preschool; 2) determine the extent to which the alignment between preschool and kindergarten classrooms shape children’s social and behavioral development; and 3) assess whether the development of children from different subgroups is differentially shaped by aspects of the classroom environment. This proposed work will generate much needed evidence across multiple data sources to provide insight into the ways in which children’s early educational experiences contribute to their development of key social and behavioral skills that lay the foundation for long-term development (Heckman, 2006). Such an understanding is of increasing importance given the growing investments in early educational opportunities in the U.S.