Background: Three different explanations have been given for the observation that adolescent gang members report more delinquent behaviour than their counterparts who do not affiliate with gangs: a) adolescents who commit more crimes join gangs (selection hypothesis); b) gang membership facilitates deviant behaviour (facilitation hypothesis); c) selection and facilitation work interactively (enhancement hypothesis). The aim of this study was to test these hypotheses, while controlling for self-reported delinquency, friends' delinquency, and individual as well as family characteristics. Method: The sample included 756 boys first assessed when they attended kindergarten in disadvantaged areas of Montreal. Gang membership was assessed at the ages of 14, 15 and 16 years. Delinquency and drug use data were collected from self-reports and court files at the same ages. Results: Gang members displayed far higher rates of delinquent behaviour and drug use than non-gang members. The results support the facilitation model for transient gang members (i.e., youths in a gang during only one of the three periods considered) and the enhancement model for stable gang members (i.e., youths in a gang for at least two of the periods considered), for person and property offenses. The association between gang membership and delinquency persisted after introducing the control variables. Additional analyses showed that the effect associated with belonging to a gang was beyond that of simply having delinquent friends. Conclusion: Preventing the creation and participation in such gangs should reduce the frequency of antisocial behaviour during adolescence.