Data on the number, structure, and functioning of delinquent gangs are hard to obtain, and no national-level agency systematically collects and reports gang-related information. Nonetheless, what data are available suggest that males outnumber female gang members 20 to 1 and that delinquent gangs are located in almost all 50 States. In a 1989 survey, 35 cities reported 1,439 gangs, and California, Illinois, and Florida had substantial gang concentrations. The history of gang behavior and violence is traced, and causes and motivations of gang aggression are examined. Psychological foundations of delinquent gangs are discussed in terms of delinquent youth, hyperadolescents, the group nature of gangs, and the effect of gangs on and interaction with the community. Appropriate intervention strategies are explored, including individual-level, community-level, and State-level intervention; judicial and therapeutic approaches; and gang outreach and group-level intervention. Various programs and levels of intervention are described. Particular attention is paid to environmental enhancers of violence, qualities of gang members, theories of delinquent behavior, and gang communications and support networks. Professionals concerned with any aspect of the delinquent gang phenomena (research, theory, prevention, or rehabilitation) are urged to recognize the desirability of including gang youth as collaborators in such efforts. 673 references, 11 tables, and 1 figure