This study explores how gang membership transitions among adolescents are related to changes with regard to peers, conventional social bonds and problem behaviour. The data come from two longitudinal studies, one conducted in the United States (the Rochester Youth Development Study) and one in the Netherlands (the NSCR School Study). In both countries, gang membership appears to be relatively short-lived, with one year as the modal length of gang membership. Also, in both countries, the results show a consistent pattern in which joining a gang is related to an increasing exposure to negative peer influences, a weakening of conventional bonds and increasing levels of delinquency and substance use. Leaving a gang is associated with the opposite pattern of change.