The influence of using substances with friends on future individual use was examined in the context of parental monitoring rules and the ecology of peer activities. A 1-year longitudinal study design included a combined sample of North Italian and French Canadian adolescents (N = 285, 53% girls, M = 14.25 years). Data analyses were conducted using structural equation modeling and multiple regression analyses. As expected, the covariation between parental monitoring and adolescent substance use was mediated by "co-use" with friends. Moreover, the relation between substance use with friends and individual substance use was moderated by parental monitoring rules and the peer activity context. Specifically, the relation between substance co-use with friends and individual substance use was stronger when the level of parental monitoring rules was low and when friends spent their time together primarily in unstructured contexts such as on the street or in park settings. These findings underline the importance of adults' use of rules to monitor adolescents prone to substance use, and the role of context in facilitating or reducing peer influence. (Contains 2 figures and 5 tables.)