The article provides a general definition of gangs and discusses different kinds of gang activity, including recent data on the role of drugs in gangs. It examines gangs from a developmental perspective, focusing on reasons why some youngsters may join gangs (including risk factors), the developmental needs that gangs may meet for some young people, and the role of parents and families in gang activity. In addition, the chapter discusses: the role of gender and ethnicity in gangs and gang-member activity; factors associated with successful prevention and intervention programs; programs that have shown iatrogenic effects; some of the macrosocial or individual factors that appear to preclude youth involvement in prosocial settings or prohibit attachment to school and other institutions; and why socially appropriate groups have lost their appeal for many youth. Tables, references