水肺是一种辅助潜水者在水中呼吸的器具。水肺是在1943年，由雅克-伊夫·库斯托和埃米尔·加尼昂共同发明的。现代的水肺系统最少由四个部分组成，包括：浮力调节设备、呼吸调节器、气瓶和配重系统。调节器以活栓操控，将压缩气体由高压状态，转化成可供人体正常呼吸的压力。密闭式循环水肺是氧气和稀有气体中的氦混合充入气瓶中的高氧水肺潜水方式，可供大深度潜水呼吸。

潜水员所使用的气瓶不称为氧气筒。氧气筒(100%氧气)是属于医疗级的气体。水肺使用的称之为气瓶(79%氮气+21%氧气)，气瓶里装的是压缩后的空气，并非纯氧气。

Aqua-Lung was the original English name of the first open-circuit, self-contained underwater breathing apparatus (or "SCUBA") to reach worldwide popularity and commercial success. This class of equipment is now commonly referred to as a diving regulator or demand valve. The Aqua-Lung was invented in Paris during the winter of 1942–1943 by two Frenchmen: the engineer Émile Gagnan and Naval Lieutenant ("lieutenant de vaisseau") Jacques Cousteau. It allowed Cousteau and Gagnan to film and explore more easily underwater.