Physiologists and evolutionary biologists have traditionally investigated different but complementary aspects of biological phenomena. Homeostasis, the maintenance of approximately constant conditions in bodily fluids, has been the purview of physiology. Evolutionary insights can, however, deepen our knowledge of homeostatic regulatory mechanisms. Bioenergetics has been of central concern to both physiology and evolution. Physiologists have been interested in the energetic content of foods, in the metabolic transformations of the energy derived from foods, and in the energetic costs of physiological processes, while evolutionary life history theory addresses the ways that organisms have evolved to acquire and allocate metabolic energy throughout their life course. Engineering control theory highlights the limitations as well as the benefits of different regulatory mechanisms and so helps to explain why we have evolved multiple integrated and cooperative homeostatic mechanisms. The physiological responses to pregnancy illustrate the ways in which an evolutionary perspective enriches our understanding of homeostasis.