a group of persons, usually youths, who share a common identity and who generally engage in criminal behaviour. In contrast to the criminal behaviour of other youths, the activities of gangs are characterized by some level of organization and continuity over time. There is no consensus on the exact definition of a gang, however, and scholars have debated whether the definition should expressly include involvement in crime. Some gangs, but not all, have strong leadership, formalized rules, and extensive use of common identifying symbols. Many gangs associate themselves with a particular geographic area or type of crime, and some use graffiti as a form of nonverbal communication.

Gang behaviour may be more universal and ancient than is commonly thought. Semiformalized groups of young adults have existed throughout recorded history, and gangs in some ancient societies engaged in very much the same kind of activities as those associated with gangs today. The speeches of the Roman (ancient Rome) orator Marcus Tullius Cicero (Cicero, Marcus Tullius), for example, contain references to armed bands of young men, led by Publius Clodius (Clodius Pulcher, Publius) and Titus Annius Milo (Milo, Titus Annius), who engaged in fights and disrupted Roman politics for years. Violent street gangs—which generated the same kind of concern as their modern-day successors—also can be identified in the 17th and 18th centuries; the Mohocks of Georgian England were feared in their time much as the Crips and Bloods of Los Angeles were in the 1990s.

Gangs flourished in colonial America, and the story of the American Revolution cannot be understood without stressing the activism of Boston's violent street gangs, which were allied with the patriot leadership. This pattern of alliance between criminal and political structures would characterize the history of American gangs well into the 20th century. In the classic model of the political machine, street gangs served to intimidate opponents, mobilize the vote, and recruit future leaders for both politics and the criminal underworld. From the 19th century onward, gangs in the United States were most active in urban centres, such as Philadelphia, which had over 100 street gangs in 1870, and Chicago, which had in excess of 1,000 gangs in the 1920s. Often, gangs operated to defend and assert the interests of ethnic groups new to the local society or region; thus, the most visible and violent gangs in each period had their roots in the latest wave of immigration. For example, Irish gangs were prevalent in the United States in the mid-19th century, followed by Jewish gangs in the early 20th century, and Asian and Latino gangs in the late 20th and early 21st century.

Gangs largely faded from the American landscape during the Great Depression and World War II only to reemerge in the late 1950s. In the 1980s and '90s, some U.S. law-enforcement officials contended that gangs posed a greater threat to public safety than ever before. Crime statistics in the 1990s suggested that gangs were responsible for a large percentage of homicide cases and for about two-fifths of illegal drug sales in urban areas. By the turn of the 21st century, researchers estimated that there were 750,000 active gang members in the United States participating in roughly 25,000 gangs. Gangs were documented in every state, in large and small cities, and in urban and rural counties, though the highest levels of gang-related crime occurred in urban centres, particularly Los Angeles and Chicago.

Significant gang activity also has taken place in Great Britain, Germany, The Netherlands, and France, as well as in Russia and the countries of eastern and central Europe following the dissolution of the Soviet Union. Across the globe, youth gangs exist wherever there are cities, and they often operate in association with adult organized-crime syndicates. In the late 20th century, gangs appeared in Africa and Latin America. Gangs in Asia—particularly the Tongs, the Chinese Triads, and various secret societies—can be linked to similar groups in the United States. Although some works have compared American and European gangs (e.g., The Eurogang Paradox 【2001】), comparatively little research has been done on gangs outside the United States.

The U.S. government estimated that at the turn of the 21st century nearly half of all gang members in the country were Hispanic, about one-third were African American, about one-eighth were white, and about one-twentieth were Asian. Research has consistently indicated that fewer than one-tenth of gang members are female, though some surveys suggested that females account for more than one-fourth of gang membership. Few gangs, however, are female dominated. About two-fifths of gang members are under the age of 17, and almost nine-tenths are under 25.

Gang involvement seems to be related to the gang member's relationships to the social institutions that are most important to all youths—family, schools, peer groups, and the labour market. Thus, gang members are more likely than other youths to come from broken homes, to perform poorly in school, to have peers who are inclined to engage in criminal behaviour, and to be unemployed or underemployed. The criminal justice system is an important intervening institution for many gang members, but, because it may generate negative attitudes toward the police and legitimate society, its effect is not always positive. Research also has shown that an individual's criminal activity increases when he joins a gang and decreases to pregang levels when he withdraws from gang activity.

Significant effort and resources have been directed toward decreasing gang membership and activity. Studies have suggested that comprehensive community-based programs are most effective. Successful programs generally have integrated strategies of community mobilization, youth opportunities, social intervention, suppression, and organizational change. Gang outreach programs that involve street workers in conjunction with social opportunities such as jobs and education have proved fairly successful in some American cities. Suppression has generally involved arrest and prosecution and has been shown to have limited utility. Organizational-change and community-mobilization strategies attempt to bring resources to gang problems in a more effective manner by attempting to meet the specific needs of youths and their families.