Obesity, which represents the most common form of malnutrition encountered in small animal practices, is mainly the result of overeating. In other words, the energy intake of the obese animal has exceeded at some stage its energy requirement leading to an excessive storage of fat. To overcome such a phenomenon, it is important to define what are the exact energy requirements of dogs and cats. The daily energy requirement of an animal should correspond to the sum of its normal resting energy expenditure, its thermogenesis, and its energy requirement associated with its physical state. The former is affected by intrinsic factors such as body size, body conformation and body composition as well as by environmental parameters (ambient temperature). The latter is affected by the physiological state of the animal (growth, gestation/lactation, ageing) and/or by the amount and type of its activity. The effects of all these factors on the individual energy requirements of dogs and cats will be reviewed here. More scientific information on energy requirements of dogs and cats is clearly necessary to avoid problems of over-or under-feeding. However, there will still be situations where animals become obese because the total energy intake is underestimated. For example, owners often forget to take into account the energy contribution of food eaten outside the regular feeding time. These situations will be discussed because of their importance in the incidence of obesity in pets.