The paper discusses a modular approach to power and total-energy systems for undersea missions. Vehicular, portable, and stationary power plants of various power levels (1–100 kwe) and duration (10–1000 and > 1000 hr) are considered. Both dynamic (open and closed Brayton and Rankine cycles) and static (battery, fuel cell, and thermoelectric) systems utilizing chemical and nuclear heat sources are compared. The comparison is based on a rating of a number of criteria for which weighing factors have been selected, e.g., weight, size, reliability, serviceability, applicability, development time, and cost. The paper indicates that several types of power systems will be necessary for undersea missions. Isotope-thermoelectrics are well suited for extremely low-power-level, long-duration missions. Batteries appear satisfactory power sources for low kilowatt hours, chemical-dynamic and fuel cell for intermediate kilowatt hours, and isotope-dynamic for high kilowatt hours. The dynamic systems have the advantage that they can be used with several heat sources, (e.g., chemical, isotope, and reactor). Recommendations are made for component development to allow early availability of power sources for undersea missions.