Evidence for the regulation of body energy is reviewed from the homeostatic perspective of Claude Bernard and Walter Cannon. The complementary roles of food intake and energy expenditure in the maintenance and defense of energy balance are considered. Particular attention is paid to the roles adjustments in energy expenditure play in this process and to recent investigations identifying their metabolic underpinnings. This is followed by a consideration of the many newly identified signals of body energy status and the pathways and feedback loops they utilize to inform the central regulating system. Finally, various naturally occurring and experimentally induced alterations in the regulated level of body energy are described and discussed. It is concluded that, though early investigators did not expressly consider energy a regulated feature of the milieu interieur, more recent research has provided a sound basis for judging the regulation of body energy to be another homeostatic process.