The goals of this chapter are to discuss the theories and evidence concerning the roles of gendered-peer interactions and relationships in children's lives at school. We begin by discussing the tendency of boys and girls to separate into same-sex peer groups and consider the theories and evidence concerning how gender segregation occurs and how peers influence children's learning and development. We then turn to the important and understudied question of why some children have more exposure to same-sex peers than others. We consider factors that contribute to variability in children's experiences with gender segregation such as the types of schools children attend and the kinds of classroom experiences they have with teachers. Finally, we review new evidence concerning the cognitive and affective factors that illustrate that children are actively involved in constructing the social world that surrounds them.