Energetics is the study of how organisms obtain and use energy from the environment. Humans obtain energy by consuming plants and other animals and use that energy to support growth, physical activity, bodily maintenance, survival, and reproduction. Foods differ markedly in energy density, that is, the energy available per unit weight. Human energy intake depends not only on the types of foods available in different environments but also on economic and sociocultural constraints that may limit access to food. Energy expenditure is driven by the physiological need to maintain organ function (the basal metabolic rate) as well as by physical activity, a highly variable factor that is also influenced by the demands of work and caregiving. For females, gestation and lactation are especially costly aspects of energy expenditure. Insufficient energy intake, even at low magnitudes, can rapidly lead to negative impacts on the capacity to work and to reproduce. Important anthropological questions regarding human energetics include how natural selection shapes the physiological and behavioral allocation of energy throughout the life cycle and how patterns of energy intake and energy expenditure vary both within and between populations.