A gang is a group or society of associates, friends, or members of a family with a defined leadership and internal organization that identifies with or claims control over territory in a community and engages, either individually or collectively, in illegal, and possibly violent, behavior, with such behavior often constituting a form of organized crime.

The word gang derives from the past participle of Old English gan, meaning 'to go'. It is cognate with Old Norse gangr, meaning 'journey'. While the term often refers specifically to criminal groups, it also has a broader meaning of any close or organized group of people, and may have neutral, positive or negative connotations depending on usage.

In discussing the banditry in American history, Barrington Moore, Jr. suggests that gangsterism as a "form of self-help which victimizes others" may appear in societies which lack strong "forces of law and order"; he characterizes European feudalism as "mainly gangsterism that had become society itself and acquired respectability through the notions of chivalry".

The 17th century saw London "terrorized by a series of organized gangs", some of them known as the Mims, Hectors, Bugles, and Dead Boys. These gangs often came into conflict with each other. Members dressed "with colored ribbons to distinguish the different factions." During the Victorian era, criminals and gangs started to form organizations which would collectively become London's criminal underworld. Criminal societies in the underworld started to develop their own ranks and groups which were sometimes called families, and were often made up of lower-classes and operated on pick-pocketry, prostitution, forgery and counterfeiting, commercial burglary, and money laundering schemes. Unique also were the use of slangs and argots used by Victorian criminal societies to distinguish each other, like those propagated by street gangs like the Peaky Blinders.

In the United States, the history of gangs began on the East Coast in 1783 following the American Revolution. Gangs arose further in the United States by the middle of the nineteenth century and were a concern for city leaders from the time they appeared. The emergence of the gangs was largely attributed to the vast rural population immigration to the urban areas. The first street-gang in the United States, the 40 Thieves, began around the late 1820s in New York City. The gangs in Washington D.C. had control of what is now Federal Triangle, in a region then known as Murder Bay. Organized crime in the United States first came to prominence in the Old West and historians such as Brian J. Robb and Erin H. Turner traced the first organized crime syndicates to the Coschise Cowboy Gang and the Wild Bunch. Prohibition would also cause a new boom in the emergence of gangs; Chicago for example had over 1,000 gangs in the 1920s.

Outside of the US and the UK, gangs exist in both urban and rural forms, like the French gangs of the Belle Époque like the Apaches and the Bonnot Gang. Many criminal organizations, such as the Italian Cosa Nostra, Japanese yakuza, Russian Bratva, and Chinese triads, have existed for centuries.

Gangs, syndicates, and other criminal groups, come in many forms, each with their own specialties and gang culture.

One of the most infamous criminal gangs are Mafias, whose activities include racketeering and overseeing illicit agreements. These include the Sicilian Cosa Nostra and the Italian–American Mafia. The Neapolitan Camorra, the Calabrian 'Ndrangheta and the Apulian Sacra Corona Unita are similar Italian organized gangs. Outside of Italy, the Irish Mob, Japanese yakuza, Chinese triads, British firms, and Russian Bratva are also examples.