Submarines, aircraft, and manned space vehicles require oxygen for human respiration. Onboard generation through the electrolysis of water is a practical means of obtaining this oxygen. Proton exchange membrane cells have been in production for many years. Existing system designs are based on maintaining the oxygen and hydrogen sides of the cells at nearly equal pressures. Recent technical advances allow high differential pressure cell stacks, which permit direct discharge of one gas at low or ambient pressure while supplying the other gas at high pressure. This capability simplifies system and component designs and results in improved reliability, safety, and operability. This paper updates the development status of high differential pressure SPE ® electrolysis cells and describes systems for applying this technology to submarine and aircraft applications.