Peer delinquency and unstructured socializing have been identified as important correlates of delinquency and substance use. This state-of-the-art review explicates research into these associations to identify important trends in the literature and directions for future research. A search of the criminological literature and literatures of allied disciplines was executed to identify studies that have examined the potential influence of peer delinquency and unstructured socializing on delinquency and substance use. The review highlights the theoretical underpinnings of the two constructs, issues of measurement quality, the generality of effects on delinquency and substance use, advances in the respective literatures, and important remaining gaps for future research to fill. While considerable attention has been given to studying the potential influence of peer delinquency and unstructured socializing on delinquency and substance use, there remain a number of ways in which these literatures can be advanced to provide a more complete understanding of the relevance of these constructs for the etiology of delinquency and substance use. •Review summarizes the literature linking peer and individual delinquency.•Review summarizes the literature linking unstructured socializing to delinquency.•Quality of measurement strategy is central to both literatures.•Associations are frequently moderated by other key theoretical variables.•Several important research questions require additional empirical scrutiny.