The features, processes, and effects of children's experiences with their peers exist on multiple levels of social complexity and intersect with many other developmental domains. Peers are implicated in several theoretical accounts of multiple development domains, including the models proposed by Piaget, Sullivan, the social learning theorists, Vygotsky, ethologists, and the symbolic interactionists. The extensive data base on peer relations points to the diverse set of positive and negative experiences that children and adolescents can have with their peers and to the breadth of the processes that account for peer effects on multiple forms of outcome including behavior (e.g., aggression), affect (e.g., depression, anxiety), school performance, self‐perceptions, moral perspectives, and physical and mental health. The chapter reviews the effects of several experiences including acceptance and rejection, exclusion, friendship, victimization, popularity, and experiences within groups. Specific attention is devoted to variation in processes and effects as a function of culture and gender.