Humans vary substantially in the magnitude of fat-free mass and fat mass, both within and across populations. These two traits change with age, during growth as well as after adulthood, and also differ between the sexes. A proportion of variability in body composition can be attributed to genetic factors, but diverse ecological stresses during the life course also contribute, and human body composition is undergoing major secular trends. Geographical patterns in body composition variability appear to derive from a range of stresses related to factors such as climate, pathogens, energy insecurity, and economic development. However, most data still relate to the body mass index, a marker of nutritional status rather than of body composition, hence accurate data on the global variability of human body composition and on secular trends therein remain lacking.