The influence of deviant peers on youth behavior is of growing concern, both in naturally occurring peer interactions and in interventions that might inadvertently exacerbate deviant development. The focus of this special issue is on understanding the moderating and mediating variables that account for peer contagion effects in interventions for youth. This set of nine innovative papers moves the field forward on three fronts: (1) Broadening the empirical basis for understanding the conditions under which peer contagion is more or less likely (that is, moderators of effects); (2) Identifying mechanisms that might account for peer contagion effects (mediators); and (3) Forging the methodological rigor that is needed to study peer contagion effects within the context of intervention trials. We propose an ecological framework for disentangling the effects of individuals, group interactions, and program contexts in understanding peer contagion effects. Finally, we suggest methodological enhancements to study peer contagion in intervention trials.KEY WORDS: peer contagion; peer influences; deviant peers.