Buddy breathing is a rescue technique used in scuba diving "out-of-gas" emergencies, when two divers share one demand valve, alternately breathing from it. Techniques have been developed for buddy breathing from both twin-hose and single-hose regulators, but to a large extent it has been superseded by safer and more reliable techniques using additional equipment, such as the use of a bailout cylinder or breathing through a secondary demand valve on the rescuer's regulator.

Running out of breathing gas most commonly happens as a result of poor gas management, but it can also happen due to unforeseen exertion, stress, or breathing equipment failure. Equipment failure resulting in the loss of all gas could be caused by failure of a pressure retaining component such as an O-ring or hose in the regulator or, in cold conditions, a freezing of water in the regulator resulting in a freeflow from the demand valve. The need for buddy breathing can be avoided by use of alternative techniques and equipment.

Buddy breathing originated from military diving following a prohibition on the training and practice of free ascents. Minor variations on the basic technique were taught by different agencies at different times, and different techniques were needed for single hose and twin hose regulators. As buddy breathing has been found to require more intensive practice than is usually provided during entry level training to be reliably used in a real emergency without endangering the buddy, and more reliable alternatives are affordably available, the procedure has been deprecated by most recreational diver training agencies in favour of more reliable and safer alternatives which are quicker to learn, and where appropriate, emergency swimming ascent.

The term has also been used for air sharing between scuba divers using an octopus demand valve, and between firefighting breathing apparatus.

Buddy breathing is a procedure for emergency air sharing between two scuba divers which was developed as a response to an out-of-gas scuba diving emergency. An out-of-gas incident underwater is a life-threatening emergency. It may occur with very little warning and requires prompt and appropriate response of restoring access to safe breathing gas while it is possible. This implies either reaching the atmospheric free surface after an emergency ascent, where air is available in unlimited amounts, or to quickly gain access to some other source of suitable breathing gas while underwater before terminating the dive. For a scuba diver who is not carrying a suitable redundant breathing gas supply, this will be gas carried by another diver in the immediate vicinity, and the most likely source is a dive buddy, another scuba diver, who by agreement accompanies the diver closely enough for the pair to be able to assist each other in any reasonably foreseeable emergency. There are several ways that breathing gas may be shared between divers, and buddy breathing is the only one of them that only uses the most basic scuba equipment. Out-of-gas incidents most commonly occur due to poor gas management, such as diving too deep or for too long, or expending too much effort and not sufficiently monitoring gas consumption, but it can also be caused by equipment failure leading to gas loss from leaks or freeflows.

Buddy breathing is usually initialised by the diver with the "out of air" emergency signalling this to another diver by a standard hand signal of a flat hand drawn across the throat or moved towards and away from the throat. This is followed by a "give me air" signal made by holding the fingers and thumb of one hand together, pointing them at the mouth and making repeated movements of the finger tips towards the mouth.

The donor is expected to take a good breath and pass their demand valve (or mouthpiece in the case of a twin-hose regulator) to the recipient, who is expected to take two good breaths and pass it back. In reality the recipient will often take more than two breaths, and the donor should expect this and relax to minimize metabolic rate. The donor should retain a good grip on the demand valve throughout the buddy breathing process, as a panicked recipient may fail to give it back. A reasonably reliable way of keeping control of the demand valve is to grasp the hose firmly in the fist where it connects to the demand valve. This provides good control but allows the recipient to use the purge button if needed.

A pattern of two breaths per diver should be established as soon as possible and then the dive is terminated and the ascent started as soon as possible, as air consumption from the shared cylinder while buddy breathing is usually more than double the normal rate, because the divers are likely to be stressed. Most demand valves will only drain correctly if the exhaust valve is at the bottom which puts the hose to the user's right, and in these cases the divers should align themselves to allow convenient movement of the demand valve from donor to recipient and back. When the divers need to ascend or swim horizontally while buddy breathing, it requires co-ordination and some skill, which is best practiced in a low-hazard environment. During ascent it is important not to hold one's breath as this could allow the gas in the lungs to expand sufficiently to cause lung over-expansion injury. The standard way of preventing this is to require the diver to be constantly blowing bubbles from the mouth when not actively inhaling, which keeps the airway open. The rate of bubble blowing, however, should not be so high as to leave no air to purge the demand valve before taking the next breath.