The recent explosion in gang research has highlighted the importance of consistent definitions for gang affiliation and gang-related crime. Definitional questions have assumed greater significance in the wake of broad-ranging prevention and intervention strategies. In this article, the authors utilize a sample of approximately 6,000 middle-school students to examine the youth gang phenomenon using five increasingly restrictive membership definitions. The least restrictive definition includes all youth who claim gang membership at some point in time. The most restrictive definition includes only those youth who are current core gang members who indicate that their gang has some degree of organizational structure and whose members are involved in illegal activities. The authors examine the differentially defined gang and nongang youths on various demographic characteristics, theretical factors, and levels of self-reported crime. The authors also address the theoretical and policy implications of shifting definitions of gang membership.