Almost five million children attend preschool in the United States each year. Recent attention has been paid to the ways in which preschool classrooms shape children's early language development. This article discusses the importance of peers and classroom composition through the lens of age and socioeconomic status and the implications for children's early learning and development. The direct and indirect mechanisms through which classroom peers may shape each other's language development are also discussed. The paper focuses on exposure to peer language and engagement with peers, along with teachers' classroom practices. Conclusions focus on the ways in which teachers can ensure that children in classrooms of different compositions reap the maximum benefit, along with implications for research, policy, and practice.