Book excerpt: This report presents an overview of gang research and programs in the United States and examines how gangs form and why youth join them. It is based on information on research findings and prevention strategies disseminated by the Office of Juvenile Justice and Delinquency Prevention as part of its comprehensive anti-gang initiative. This report describes how community members can start assessing their gang problems and enhance prevention and intervention activities to help prevent delinquency and gang violence. It identifies promising and effective programs for gang prevention. Illustrations. This is a print on demand edition of an important, hard-to-find publication.