The reciprocal relation between deviant friendships and substance use was examined from early adolescence (age 13-14) to young adulthood (age 22-23). Deviance within friendships was studied using direct observations of videotaped friendship interactions and global reports of deviant interactions with friends as well as time spent with friends. Substance use was assessed through youth self-report at all time points. Multivariate modeling revealed that substance use in young adulthood is a joint outcome of friendship influences and selection process. In addition, substance use appears to influence the selection of friends in late adolescence. Findings suggest that effective preventions should target peer ecologies conductive to substance use that that treatment should address both the interpersonal underpinnings and addiction processes intrinsic to chronic use, dependence, and abuse.