During the period studied, the Gang Control Unit investigated 217 gang incidents and 95 events attributed to groups. The Unit defines a gang as a highly developed organization with a large membership who identify with the group and who have a particular circumscribed territory. A delinquent group, on the other hand, is perceived as a small group who join together sporadically and spontaneously violate the law. The differences between gang activities and group activities are described. The Gang Control Unit obtains information through routine patrols, police referrals, citizen reports, police radio broadcasts, and contacts with gang members. Analysis of the offenses committed by gangs indicates that the majority involve homicide, rape, aggravated assault, and battery, and simple assault, whereas youth groups tend to engage in property offenses. Gangs also inflict more serious injury to the community than groups do. The typical gang member in Philadelphia is between 15 and 17 years old, male, black, and lives in a low-income area. In general, gang participants tend to be more homogeneous than their group counterparts. Victims of gang violence in 90 percent of the reports were either gang members, delinquents, acquaintances of the offenders, or of the same age and race as the perpetrators. Approximately 80 percent of the gang events were triggered by an explicit motive as compared to 68 percent of the group incidents. These research findings provide the basis for a revised concept of internecine delinquency characterized by interacting victim-offender systems which revolve around territorial defense. In conclusion, the sociophysical segregation of lower-class black youth produces disproportionate territorial attachment which in turn explains patterns of gang activity. The article contains several statistical tables and 60 footnotes. For related entries, see NCJ 72869-70 and 72872-74.