The lack of compact, efficient, human compatible, lightweight power sources impedes the realization of machine-enhanced human endeavor. Electronic and communication devices, as well as mobile robotic devices, need new power sources that will allow them to operate autonomously for periods of hours. In this work, a personal power system implies an application of interest to an individual person. The human-compatible gravimetric energy density spans the range from 500 to 5000 Wh/kg, with gravimetric power density requirements from 10 to 1000 W/kg. These requirements are the primary goals for the systems presented here. The review examines the interesting and promising concepts in electrochemical, thermochemical, and biochemical approaches to small-scale power, as well as their technological and physical challenges and limitations. Often it is the limitations that dominate, so that while the technology to create personal autonomy for communications, information processing and mobility has accelerated, similar breakthroughs for the systems powering these devices have not yet occurred. Fuel cells, model airplane engines, and hummingbird metabolism, are three promising examples, respectively, of electrochemical, thermochemical, and biochemical power production strategies that are close to achieving personal power systems' power demands. Fuel cells show great promise as an energy source when relatively low power density is demanded, but they cannot yet deliver high peak powers nor respond quickly to variable loads. Current small-scale engines, while achieving extraordinary power densities, are too inefficient to achieve the energy density needed for long-duration autonomous operation. Metabolic processes of flying insects and hummingbirds are remarkable biological energy converters, but duplicating, accelerating, and harnessing such power for mobility applications is virtually unexplored. These challenges are significant, and they provide a fertile environment for research and development. (c) 2005 Elsevier Ltd. All rights reserved.