This articles looks at the causes of diabetes mellitus (DM) in both cats and dogs, including genetic and environmental factors. Genetic factors in type 2 feline DM are suggested by the overrepresentation of Burmese cats. Age, obesity, male gender, neutering, physical inactivity and living indoor are environmental predisposing factors to DM in cats and, therefore, low-carbohydrate, high-protein diets could prevent it in cats at risk. In dogs, at least 50% of type 1 DM cases are caused by immune destruction of beta-cells. High-fat diets are potentially associated with pancreatitis, that causes around 28% of canine DB, but obesity doesn’t seem to be a risk factor.