Underwater breathing apparatus is equipment which allows the user to breathe underwater. The three major categories of ambient pressure underwater breathing apparatus are:

Two other types may also be identified:

Underwater breathing apparatus can be classified as open circuit, semi-closed circuit, (including gas extenders) or closed circuit (including reclaim systems), based on whether any of the exhaled gas is recycled, and as self-contained or remotely supplied (usually surface-supplied, but also possibly from a lock-out submersible or an underwater habitat), depending on where the source of the primary breathing gas is located. It may also be classified as ambient pressure or atmospheric pressure systems, and by purpose, between diving equipment and escape equipment.

A scuba set is any self-contained breathing apparatus that is carried entirely by an underwater diver and provides the diver with breathing gas at the ambient pressure. Although strictly speaking the scuba set is only the diving equipment which is required for providing breathing gas to the diver, general usage includes the harness by which it is carried, and those accessories which are integral parts of the harness and breathing apparatus assembly, such as a jacket or wing style buoyancy compensator and instruments mounted in a combined housing with the pressure gauge. In open-circuit demand scuba, the diver expels exhaled air to the environment, and requires each breath be delivered on demand by a diving regulator, which reduces the pressure from the storage cylinder. The breathing air is supplied through a demand valve when the diver reduces the pressure in the demand valve during inhalation.

A diving rebreather recirculates the breathing gas already used by the diver after replacing oxygen used by the diver and removing the carbon dioxide metabolic product. Rebreather diving is used by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable. The main advantages of rebreather diving are extended gas endurance, and lack of bubbles. Rebreathers are generally used for scuba applications, but are also occasionally used for bailout systems for surface supplied diving. Rebreathers are more complex to use than open circuit scuba, and have more potential points of failure, so acceptably safe use requires a greater level of skill, attention and situational awareness, which is usually derived from understanding the systems, diligent maintenance and overlearning the practical skills of operation and fault recovery.

The essential aspect of surface-supplied diving is that breathing gas is supplied from the surface, either from a specialized diving compressor, high-pressure cylinders, or both. In commercial and military surface-supplied diving, a backup source of breathing gas is generally required, by regulatory legislation of code of practice, to be present in case the primary supply fails. The diver may also wear an emergency gas supply cylinder called a "bail-out bottle," which can provide self-contained breathing gas in an emergency. The surface-supplied diver is much less likely to have an "out-of-air" emergency than a scuba diver as there are normally two alternative breathing gas sources available. Surface-supplied diving equipment usually includes communication capability with the surface, which improves the safety and efficiency of the working diver.

Surface-supplied diving includes diving using an umbilical with gas supply hose, lifeline strength member and communications cable, using a helmet or full-face mask, and diving with a simple air line, also known as hookah equipment., though regulatory legislation may in some jurisdictions exclude air line equipment from their definition.

Surface-supplied air is provided on an open-circuit system, as it is simpler and more economical than recycling, and when supplied from the ambient atmosphere by a low pressure compressor, is practically unlimited.