The first commercially successful scuba sets were the Aqualung twin hose open-circuit units developed by Emile Gagnan and Jacques-Yves Cousteau, in which compressed air carried in back mounted cylinders is inhaled through a demand regulator and then exhaled into the water adjacent to the tank.[3] The single hose two stage scuba regulators of today trace their origins to Australia, where Ted Eldred developed the first example of this type of regulator, known as the Porpoise, which was developed because patents protected the Aqualung's twin hose design. The single hose regulator separates the cylinder from the demand valve, giving the diver air at the pressure at their mouth, not that at the top of the cylinder.

The open circuit compressed air systems were developed after Cousteau had a number of incidents of oxygen toxicity using an oxygenrebreather, in which exhaled oxygen is passed through an absorbent chemical to remove carbon dioxide before being breathed again. Modern versions of rebreather systems (both semi-closed circuit and closed circuit) are available, and form the second main type of scuba unit, mostly used for technical and military diving