Racial/ethnic and economic school segregation represent pervasive instantiations and perpetuators of educational inequity in the United States. Although the U.S. is becoming increasingly racially and ethnically diverse, schools are racially/ethnically isolated. Rising income and wealth inequity are also exacerbating economic school segregation. There is robust evidence linking school segregation to worse academic outcomes for children. However, far less is known about the consequences of school segregation for children's social development, particularly in elementary school, an understudied but critical period when children are most likely to experience segregation. This study merged the Segregation Index's School Segregation Database (SegIndex) and Stanford Education Data Archive 4.1 (SEDA) with the National Center for Education Statistics' Early Childhood Longitudinal Study, Kindergarten Class of 2010-11 (ECLS-K:2011) and used multi-level modeling to explore how racial/ethnic and economic school segregation at the district and county levels related to social development for a national sample of [approximately equal to]12,000 children from third through fifth grade. It also considered whether these links differed depending on children's racial/ethnic and socioeconomic backgrounds. Results of unadjusted models suggested that more district- and county-level racial/ethnic and economic school segregation were related to child reports of less perceived interest/competence in peer relationships, prosocial behavior, behavioral engagement, peer social support, and more stress. Conversely, more school segregation was related to less social anxiety. Most of these results did not endure in fully adjusted models, suggesting that these associations are confounded with other child, school, district, and or county factors. Additionally, moderation analyses suggested that there are no systematic differences in associations between school segregation and social development according to children's race/ethnicity or socioeconomic background. The inconsistent and null findings introduce questions about mechanisms linking school segregation to social development that are discussed along with future directions and policy implications. The more researchers, policymakers, and other stakeholders learn about how contexts of school segregation shape children's development, the better equipped they will be to design and support equitable integration strategies for U.S. schools. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]