Diabetes mellitus (DM) is a common disease in dogs and cats and its prevalence is increasing in both species, probably due to an increase in obesity, although only in cats has obesity been clearly identified as a major risk factor for diabetes. While the classification of diabetes in dogs and cats has been modeled after that of humans, many aspects are different. Autoimmune destruction of beta cells, a feature of type 1 DM in people, is common in dogs; however, in contrast to what is seen in people, the disease occurs in older dogs. Diabetes also occurs in older cats but islet pathology in those species is characterized by the presence of amyloid, the hallmark of type 2 DM. Despite being overweight or obese, most naive diabetic cats, contrary to type 2 diabetic humans, present with low insulin concentrations. The physiology of carbohydrate metabolism and pathogenesis of diabetes, including histopathologic findings, in dogs and cats are discussed in this chapter.