Air-independent propulsion (AIP), or air-independent power, is any marine propulsion technology that allows a non-nuclear submarine to operate without access to atmospheric oxygen (by surfacing or using a snorkel). AIP can augment or replace the diesel-electric propulsion system of non-nuclear vessels.

Modern non-nuclear submarines are potentially stealthier than nuclear submarines; although some modern submarine reactors are designed to rely on natural circulation, most naval nuclear reactors use pumps to constantly circulate the reactor coolant, generating some amount of detectable noise. Non-nuclear submarines running on battery power or AIP, on the other hand, can be virtually silent. While nuclear-powered designs still dominate in submergence times, speed, range, and deep-ocean performance, small, high-tech non-nuclear attack submarines can be highly effective in coastal operations and pose a significant threat to less-stealthy and less-maneuverable nuclear submarines.

AIP is usually implemented as an auxiliary source, with the traditional diesel engine handling surface propulsion. Most such systems generate electricity, which in turn drives an electric motor for propulsion or recharges the boat's batteries. The submarine's electrical system is also used for providing "hotel services"—ventilation, lighting, heating etc.—although this consumes a small amount of power compared to that required for propulsion.

AIP can be retrofitted into existing submarine hulls by inserting an additional hull section. AIP does not typically provide the endurance or power to replace atmospheric dependent propulsion, but allows for longer underwater endurance than a conventionally propelled submarine. A typical conventional power plant provides 3 megawatts maximum, and an AIP source around 10% of that.^{[citation needed]} A nuclear submarine's propulsion plant is usually much greater than 20 megawatts.

The United States Navy uses the hull classification symbol "SSP" to designate boats powered by AIP, while retaining "SSK" for classic diesel-electric attack submarines.

In the development of submarines, the challenge of finding satisfactory forms of underwater propulsion has been persistent. The earliest submarines were powered by hand-cranked propellers, which quickly depleted the air supply; as a result, these vessels often had to operate on the surface with hatches open or use some form of breathing tube, both of which were inherently dangerous and led to numerous early accidents. Later, mechanically driven vessels utilized compressed air, steam, or electricity, which needed to be recharged from shore or from an onboard aerobic engine.

The earliest attempt at a fuel that would burn anaerobically was in 1867, when Spanish engineer Narciso Monturiol successfully developed a chemically powered anaerobic or air independent steam engine. The engine was powered by a mixture of potassium chlorate and zinc, which reacted to generate heat and, conveniently, oxygen.

In 1908 the Imperial Russian Navy launched the submarine Pochtovy, which used a gasoline engine fed with compressed air and exhausted under water.