Future tactical aircraft will have increased capabilities that will place greater demands on their secondary power systems. Added capabilities such as low observability or internal weapons storage are being planned for without significantly increasing the aircraft's size and weight. The power system must therefore have reduced volume, weight, and complexity, while also being more reliable and maintainable. The auxiliary power unit (APU) is a critical component that must be improved to upgrade the capabilities of the power system. Increasing the APU's power density is one important way for reducing the power system's size and weight. Increased power density, however, will require a power unit operating with higher gas generator temperatures, so this condition will be the major challenge for new APU designs. To define the effect of present and future APU development, the following subjects are discussed: 1) estimation of the weight, performance, and a mission profile of a new multi-role fighter, 2) the development of APU technology to meet the anticipated requirements of this aircraft, and 3) estimation of the benefits of power system improvement by estimating the life-cycle cost reduction.