Despite the lack of a universal definition of a gang, the research presented in this paper can be applied to any youth group in which the majority of its membership engages in initiation rituals, hand signals, graffiti, drug use, and the setting of territorial boundaries for the group. It is also common for gang members to use a variety of identifying marks, such as specific clothing, tattoos, hairstyles, and language. Multiple data sources indicate that gang activity has increased dramatically over the last two decades; whether or not this increase has continued over the last few years is currently in question. If there are to be better explanations for gang participation and if an interdisciplinary theory that might predict when an individual might join a gang is to be developed, psychology must become more involved. One section of this paper focuses on Erikson's psychosocial theory ("Childhood and Society" and "Identity, Youth and Crisis"). According to Erikson, the ego develops over a predetermined set of stages that begin during a critical time period. This paper focuses on the fifth stage identified by Erikson as "Identity vs. Role Confusion." In this stage, many adolescents reject the standards imposed on them by adults and resort to "clannish" behavior that involves being cruel to others perceived as different from their group. During this stage, adolescents also seek to develop a workable identity that involves an ideology and a set of values. Under this theory, it is reasonable to suggest that joining a gang can foster the development of an identity and a clear behavioral and ideological map. This theory, however, does not sufficiently explain why some adolescents join antisocial gangs and others do not. Other sections of the paper discuss general socialization theory, social learning theory, and ecological systems theory as possibly relevant to a psychological theory for why youth join gangs. Suggestions are offered for additional research, given the sparseness of psychological research on gangs. 49 references