It is estimated that 30 to 40% of dogs and cats in the United States are overweight or obese. Pet obesity increases the risk of several diseases and disorders, including insulin resistance, diabetes mellitus, hyperlipidemia, cancers, and many others, and results in a shorter life span. Although genetics, a sedentary lifestyle, and neutering contribute to weight gain, the over-consumption of energy-dense foods is a major driver. There are many challenges to preventing pet obesity, including owner beliefs and feeding tendencies, estimation of energy density, and development of appropriate feeding guidelines. Owner education, especially as it pertains to body condition scoring and feeding guidelines, is a critical need. Feeding guidelines are required on all pet foods, but many owners either do not read them carefully and/or do not understand the importance of adjusting intake based on body condition score. Ad libitum feeding and excessive feeding of table scraps and treats contribute to overfeeding. Estimating caloric content of ingredients or pet foods may also be difficult and quite different depending on the methodology used. Numerous equations are used to estimate the caloric density of diets based on proximate analysis, but they are all based on assumptions pertaining to the digestibility and metabolism of nutrients. The use of crude fiber, a highly inaccurate method of fiber estimation, also contributes to inaccuracies in determining digestible carbohydrate content and consequent caloric density estimates. In vitro assays that estimate nutrient digestibility have been developed, but in vivo tests are by far the most accurate method by which metabolizable energy content of diets may be determined. Even when an accurate caloric density has been determined, developing accurate feeding instructions are difficult because a large variation among pet animals exists, including differences in physical activity level, age, life stage, and metabolism. Fortunately, AAFCO regulations will be requiring a calorie statement on all pet foods in the near future. Given the issues listed above, however, it is questionable whether that requirement will aid in obesity prevention or provide false hope to veterinarians and pet food professionals.