Scuba diving ("SCUBA" originally being an acronym for self contained underwater breathing apparatus, 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.

Unlike early diving, which relied either on breath-hold or on air pumped from the surface, scuba divers carry their own source of breathing gas (usually compressed air), allowing them greater freedom of movement 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. Depending on 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.

The first commercially successful scuba sets were the Aqualung open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed gas (usually air) is inhaled from a tank and then exhaled into the water adjacent to the tank. However, the scuba regulators of today trace their origins to Australia, where Ted Eldred developed the first mouth piece regulator, known as the Porpoise. This regulator was developed because patents protected the Aqualung's double hose design. It separated the cylinder from the demand valve giving the diver air at the same pressure surrounding his mouth, not surrounding the tank.