After reviewing two of the most common methods for measuring a juvenile's exposure to delinquent peers -- self-report questions about the involvement of friends in various delinquent offenses and the questioning of named friends about their delinquent behavior -- the author concludes that the measurement issue is not yet resolved. Both of the most common measurement methods result in significant correlations between having delinquent peers and being delinquent; however, the magnitude of overestimation or underestimation remains unknown. Both methods should be included in future research. The complexity of determining the causal dynamics of the influence of delinquent peers on delinquent behavior is examined, with the author arguing that research methods to date have not taken into account all the possible variables involved in interactions with peers. Regarding "precision," the author advises that more precision is needed in explaining the actual processes and mechanisms that may be responsible for the role of delinquent peers in influencing delinquent behavior. The chapter's section on recent research findings on the role of delinquent peers during childhood and adolescence focuses on research on causality, temporal order, the changing effects of delinquent peers during childhood and adolescence, and rejection and popularity in childhood and adolescence. The research challenges identified are why and how friendships and other relationships with delinquent peers evolve and why the role of delinquent peers changes during a juvenile's life course. Areas for future research are juveniles' social networks, the role of delinquent peers during the life course, and the analysis of interaction effects. 66 references