Peer interaction is examined as a source of behavior change in children and adolescents. The dynamics of peer influence are discussed in terms of six issues: characteristics of the influence source; characteristics of the child or adolescent being influenced; the relationship existing between influence source and the individual being influenced; developmental change; processes underlying peer influence; and constraints deriving from the behavior domain being examined. Research shows that contact with peers has complex effects on individual behavior, and numerous moderators and mediators must be considered in order to account for them. The discussion is linked to the papers included in this special issue.KEY WORDS: peers; peer interaction; deviance; peer relationships.