The recent concern over depletion of nonrenewable fuels revived interest in energy analysis as a means of evaluating the physical effects of human activities. Much of this concern arose out of the work of ecologists and energy systems analysts who were seeking an alternative source of values for assessing the natural resource base. Some proponents also argue that impacts on ecosystems can be expressed in energy terms. This chapter begins by discussing the stated rationale for energy analysis and its development and forms. It then covers the conceptual and practical problems in the application of these techniques, and their possible relevance in environmental assessments.