Criminologists often allude to ‘peer influence’ in explanations of crime and delinquency, but the meaning of that concept rarely receives careful attention. Companions in Crime organizes the extensive literature on peer influence and group delinquency into a coherent form for the first time. Chapters focus on the role of peers over the life course, the group nature of delinquent behavior, and the applicability of peer influence for explaining the major features of delinquent behavior. The most extensive chapter of the book examines possible mechanisms of peer influence and the evidence in favor of each. The principal thesis of Companions in Crime is that deviant behavior is predominantly social behavior, and criminologists must eventually determine the significance of that fact.