Recent years have seen increasing proportions of children attending early childhood education (ECE) programs, and growing involvement of local, state, and federal governments in funding, regulating, and directing ECE efforts. In this context, there is a growing need to delineate mechanisms to best target public resources, to improve ECE program quality, and to promote the academic and behavioral skills of young children. This project seeks to delineate the role of an understudied component of ECE contexts: the role of peers. Based on theoretical frameworks arguing for the central role of peers in affecting both children and adults, this study will specify and test a conceptual model delineated specific paths through which peer effects operate. Specifically, we hypothesize that peer cognitive and behavioral skills within a classroom will predict shifts in individual child skills through the preschool year, and will also predict shifts in teacher instructional practices, which in turn are hypothesized to support children's functioning as they transition into kindergarten. Moreover, we argue that these associations will function both within and across cognitive and behavioral realms of functioning, and that they may be moderated by initial levels of child and teacher functioning. As the first study to assess the potential for peers to affect both individual children and teachers, this research has the potential to provide essential insights into classroom composition effects, which in turn will inform ECE policies regarding targeted versus universal programs and efforts to improve ECE quality and children's development.