The chapter examines risk factors for individuals joining gangs, the relative contribution of gang members to the overall volume of crime in society, and the social processes by which membership in a street gang brings about an increased level of involvement in delinquency by gang members. In addition, the chapter compares gangs and other law-violating groups to see if gangs are qualitatively different from those groups or simply an extension of a process by which association with delinquent peers increases involvement in delinquency. The chapter confirms that gang members are clearly major contributors to the level of serious and violent crime in American society, especially while they are active gang members, and that the number of gangs has expanded massively in recent years. It follows, therefore, that one of American society's highest priorities in the effort to reduce the level of serious and violent delinquency is to develop effective intervention programs for street gangs. Tables, notes