Scientists searching for extraterrestrial life forms have recently begun to focus their attention on a group of icy bodies in the outer solar system. These satellites, particularly Europa, Enceladus, Titan and Triton, are thought to contain both organic molecules and liquid water, and, unlike the organic-rich comets and meteorites of the inner solar system, may also have suitable energy sources for simple organic chemistry to proceed. Key energy sources on the icy satellites include radioactive heating and tidal energy production driven by the interaction of the satellites with the gravitational fields of nearby bodies. Although the amount of energy generated by each of these processes is as yet unknown, modelling exercises have suggested that either may be sufficient to melt substantial parts of the satellites? interiors and provide a sub-surface ocean suitable for extraterrestrial life to develop. Understanding energy sources on icy satellites, therefore, is important for astrobiology, as well as potentially contributing to our understanding of the geomorphology and surface evolution of those bodies. This article investigates the key energy sources on the active satellites of the outer solar system, and assesses their potential for supporting life.