As anyone with an Open Water certification knows, divers need to keep track of the amount of nitrogen accumulated in their bodies under water so that they do not exceed the non-decompression limit (NDL). Sometimes keeping within the NDL can limit the length of the dive, especially in the depth range 20-30 meters or in case of repetitive dives. One way to prolong the NDL is to breath a gas mixture that contains a higher percentage oxygen and lower percentage nitrogen than regular air, known as nitrox. This course focuses on the theory behind the physiological effect of breathing nitrox, essential safety techniques, as well as proper gas analysis and tank labeling procedures. We also include an actual open water dive on a nitrox tank.