Scuba diving ("scuba" originally being an acronym for Self Contained Underwater Breathing Apparatus, although now widely considered a word in its own right) is a form of underwater diving in which a diver uses a scuba set to breathe underwater for recreation, commercial or industrial reasons.

Unlike early diving, which relied exclusively on air pumped from the surface, scuba divers carry their own source of breathing gas (usually compressed air), allowing them greater freedom than with an air line.

Both surface supplied and scuba diving allow divers to stay underwater significantly longer than with breath-holding techniques as used in snorkelling and free-diving.

According to the purpose of the dive, a diver usually moves underwater by swimfins attached to the feet, but external propulsion can come from an underwater vehicle, or a sled pulled from the surface.

Scuba diving may be performed for a number of reasons, both personal and professional. Most people begin through recreational diving, which is performed purely for enjoyment and has a number of distinct technical disciplines to increase interest underwater, such as cave diving, wreck diving, ice diving and deep diving.

Divers may be employed professionally to perform tasks underwater. Most of these commercial divers are employed to perform tasks related to the running of a business involving deep water, including civil engineering tasks such as in oil exploration, underwater welding or offshore construction.

Commercial divers may also be employed to perform tasks specifically related to marine activities, such as naval diving, including the repair and inspection of boats and ships, salvage of wrecks or underwater fishing, like spear fishing.

Other specialist areas of diving include military diving, with a long history of military frogmen in various roles. They can perform roles including direct combat, infiltration behind enemy lines, placing mines or using a manned torpedo, bomb disposal or engineering operations.

In civilian operations, many police forces operate police diving teams to perform search and recovery or search and rescue operations and to assist with the detection of crime which may involve bodies of water. In some cases diver rescue teams may also be part of a fire department or lifeguard unit.

Lastly, there are professional divers involved with the water itself, such as underwater photography or underwater filming divers, who set out to document the underwater world, or scientific diving, including marine biology and underwater archaeology.