The central characteristic of the evolution of the combat soldier in recent years is an increasingly sophisticated array of sensing, communications, and related electronics for use in battlefield situations. The most critical factor for maintaining this evolution will be the development of power supply systems capable of operating those electronics effectively for missions up to 72 hours long. To address the challenge, it is important that new approaches be sought on how to integrate and power these electronics. To assist in addressing this problem, the Army requested the National Research Council to review the state of the art and to recommend technologies that will support the rapid development of effective power systems for the future warrior. This report presents the results of that review. It provides an assessment of various technology options for different power level requirements, power system design, and soldier energy sinks. The report also describes future design concepts, focusing on low-power systems. Recommendations for technology development and system design are presented.