Most pet owners in the United States fulfill their pet’s nutritional needs through feeding commercially prepared pet foods. These are supplied with suggested feeding amounts which individual pet-parents adjust to maintain body weight and influence body composition. Significant research efforts have been expended to develop methods for estimating energy concentration of pet foods; however, even the poorest estimates of pet food energy are less variable than the variation of energy need between pets. For example, the estimate for canned cat food has an average difference of 9% and including 2 standard deviations, still had an estimate within 20% of the measured value. The variation of dogs and cats, which are living in the same environment, shows that dogs on average maintain weight consuming 121 kcal/kg^3/4 daily, while the cat uses 73. There is a greater than 2-fold difference between the dog or cat using the least or most amount of energy. Therefore, although the relationship between dietary energy and the bioenergetics of pet food starts with the energy concentration of pet food it is most significantly concerned with the energy use associated with life. Obese dogs and cats use less energy to maintain weight than do their lean counterparts. However, the amount of energy needed to maintain weight in obese pets is subject to change and may be influenced to equal or even exceed the energy needed by normal pets to maintain weight. It has been shown that when dogs were fed a food with increased amounts of fiber there was spontaneous reduction in ad libitum energy intake and repartitioning of energy away from fat so that the energy deficit was completely balanced by energy mobilization from fat. This is also the normal response to food restriction. However, in both dogs and cats a feeding program with controlled weight loss, followed by weight maintenance, when consuming a nutritionally enhanced food, resulted in a significant change in body composition and a significant change in energy needed to maintain weight. This suggests that the bioenergetics of pet obesity is established by the food being eaten, the individual pet, and the feeding regimen.