Peer influence occurs across a wide variety of behavioral domains, which is an important reason for peer-led interventions: interventions in which peers are involved in the delivery of the program. These programs are promising in combatting undesirable behaviors (e.g. risk behavior) and promoting desirable behavior (e.g. healthy lifestyle), but it was shown recently that the effectiveness of these programs is modest at best and the mechanisms underlying programs’ effectiveness are poorly understood. Research is needed that promotes understanding of the relative, cumulative, and interactive impacts of different types of peer relations and unpacks the various mechanisms underlying peer selection and influence. This has the potential to yield insights that advance theory and optimize peer-led interventions.