The United States has seen rapid proliferation of youth gangs since 1980. During this period, the number of cities with gang problems increased from an estimated 286 jurisdictions with more than 2,000 gangs and nearly 100,000 gang members in 1980 (Miller, 1992) to about 4,800 jurisdictions with more than 31,000 gangs and approximately 846,000 gang members in 1996 (Moore and Terrett, in press). An 11-city survey of eighth graders found that 9 percent were currently gang members, and 17 percent said they had belonged to a gang at some point in their lives (Esbensen and Osgood, 1997). Other studies reported comparable percentages and also showed that gang members were responsible for a large proportion of violent offenses. In the Rochester site of the OJJDP-funded Program of Research on the Causes and Correlates of Delinquency, gang members (30 percent of the sample) self-reported committing 68 percent of all violent offenses (Thornberry, 1998). In the Denver site, adolescent gang members (14 percent of the sample) self-reported committing 89 percent of all serious violent offenses (Huizinga, 1997). In another study, supported by OJJDP and several other agencies and organizations, adolescent gang members in Seattle (15 percent of the sample) self-reported involvement in 85 percent of robberies committed by the entire sample (Battin et al., 1998). This Bulletin reviews data and research to consolidate available knowledge on youth gangs that are involved in criminal activity. Following a historical perspective, demographic information is presented. The scope of the problem is assessed, including gang problems in juvenile detention and correctional facilities. Several issues are then addressed by reviewing gang studies to provide a clearer understanding of youth gang problems. An extensive list of references is provided for further review.