A bouncer (also known as a door supervisor) is a type of security guard, employed at licensed or sanctioned venues such as bars, nightclubs, cabaret clubs, strip clubs and casinos. A bouncer's duties are to provide security, to check legal age and drinking age, to refuse entry for intoxicated people, and to deal with aggressive, violent or verbal behavior or disobedience with statutory or establishment rules. They are also charged with maintaining order, and ensuring that laws and regulations are followed by all patrons.

They are civilians and they are often hired directly by the venue, rather than by a security firm. Bouncers are often required where crowd size, clientele or alcohol consumption may make arguments or fights a possibility, or where the threat or presence of criminal gang activity or violence is high. At some clubs, bouncers are also responsible for "face control", choosing who is allowed to patronize the establishment. Some establishments may also assign a bouncer to be responsible for cover charge collections. In the United States, civil liability and court costs related to the use of force by bouncers are "the highest preventable loss found within the [bar] industry", as many United States bouncers are often taken to court and other countries have similar problems of excessive force. In many countries, state governments have taken steps to professionalise the industry by requiring bouncers to have training, licensing, and a criminal records background check. In the United Kingdom, all door supervisors are required to be registered with and are licensed by the Security Industry Authority. These operatives go through a one week training regime, and are often more highly skilled than operatives without this training, as is evident by the reduced number of assaults by bouncers since the introduction of the license. Some licensed premises are obligated to use registered door supervisors as a condition of their license.

Other terms include "cooler" in the U.S. and "door supervisor" in the U.K. In U.S. bars, "cooler" is often the term for the head bouncer. The "cooler" is expected to have the same ability to respond to physical situations as the rest of the bouncers, but should also have reliable interpersonal skills that can be used to de-escalate situations without violence.^{[failed verification]}

In the 1990s and 2000s, increased awareness of the risks of lawsuits and criminal charges have led many bars and venues to train their bouncers to use communication and conflict resolution skills before, or rather than, resorting to brute force against troublemakers. However, the earlier history of the occupation suggests that the stereotype of bouncers as rough, tough, physical enforcers has indeed been the case in many countries and cultures throughout history. Historical references also suggest that the 'door supervisor' function of guarding a place and selecting who can have entry to it (the stereotypical task of the modern bouncer) could at times be an honorific and evolve into a relatively important position.

The significance of the door supervisor as the person allowing (or barring) entry is found in a number of Mesopotamian myths (and later in Greek myths descended from them), including that of Nergal overcoming the seven doormen guarding the gates to the Underworld.

In 1 Chronicles 26 of the Old Testament, the Levitical Temple is described as having a number of 'gatekeepers'—amongst their duties are "protect[ing] the temple from theft", from "illegal entry into sacred areas" and "maintain[ing] order", all functions they share with the modern concept of the bouncer, though the described temple servants also serve as holy persons and administrators themselves (it is noted that some administrative function is still present in today's bouncing in the higher position of the supervisor). Door supervisors or bouncers are usually larger persons who display great strength and size.

The Romans had a position known as the ostiarius (doorkeeper), initially a slave, who guarded the door, and sometimes ejected unwanted people from the house whose gate he guarded. The term later become a low-ranking clergy title. Plautus, in his play Bacchides (written approximately 194–184 BC), mentions a "large and powerful" doorman / bouncer as a threat to get an unwelcome visitor to leave. Tertullian, an early Christian author living mainly in the 2nd century AD, while reporting on the casual oppression of Christians in Carthage, noted that bouncers were counted as part of a semi-legal underworld, amongst other 'shady' characters such as gamblers and pimps.

Within Buddhist lore, the Nio, the twin guardian statues who decorate the entrances of temples, dojos, and various places related with religious importance, are believed to be the embodiment of wrathful deity bodhisattvas such as Vajrapani, Mahasthamaprapta, Vajrasattva, Acala, or various figures within Buddhist legend who accompanied Gautama Buddha during his journeys to spread his teachings to protect him and aid in cases needing justified force.