Using data from the author’s experience and from the literature, and using the concept of a two-compartment model, the effect of various influences on body composition are briefly examined, with emphasis on energy balance. In many situations, in humans and animals alike, changes in weight involve both lean body mass (LBM) and body fat, and usually in the same direction. Hence the two can be considered companions. However, the relative contribution of each to the total weight change varies with circumstances.