Peers provide a context in which child and adolescent development occurs. Peers may right wayward developmental trajectories, buffering the effects of deleterious risk factors. Adverse peer experiences also may distort developmental pathways, contributing to psychopathology. Numerous distinct peer constructs that signal developmental perturbations have been identified and associated with indices of psychopathology. This chapter reviews peer experiences in early childhood (i.e., social withdrawal), school‐age youth (i.e., peer status, peer victimization, and friendship), and adolescence (i.e., peer influence) that are associated with psychological symptoms. This chapter highlights developmental psychopathology approaches for understanding transactions between peer experiences and multiple development systems. New directions for research, integrating affective neuroscience theories, psychophysiology findings, and more complex cascade models of development are discussed. The peer context is highly relevant for understanding psychological and even biological functioning in youth. Ongoing research integrates findings from the peer relations literature with research in related scientific disciplines.