Undersea application of compact and lightweight closed Brayton cycle power systems currently being developed for space is discussed. Design details of a 50 kw e hydrogen-oxygen fueled system for a short-duration undersea mission of 1000 kw-hr are presented. For this same mission, a new hydrogen-oxygen injection gas turbine cycle is described. Performance characteristics of undersea long-duration 10 to 150 kw e power systems, using radioisotope and nuclear reactor heat sources, are also presented. Development status of closed Brayton cycle power conversion systems is presented.