Diabetes mellitus is the most common disorder of the endocrine pancreas in dogs and cats. The incidence in dogs is between 0.3% and 1%, and the incidence in cats is between 0.5% and 1.2%, depending on the study population, and is influenced by geographical location and type of veterinary practice (referral or primary accession).

Diabetes mellitus is a disease of middle aged to older companion animals, with peak prevalence of 7-12 years of age in dogs, and 10 – 13 years in cats. Intact female dogs, and male cats, are predisposed. Various breeds of cat and dog are overrepresented, and predisposed breeds vary with geographic area. For example, in the USA, dog breeds at increased risk include miniature Schnauzer, Samoyed, and miniature poodle, whereas in the UK, in addition to Miniature Schnauzer and Samoyed, breeds more commonly seen include Tibetan, Cairn, Border and Yorkshire Terriers and Labrador retrievers. In cats, Burmese are overrepresented in Australia, New Zealand and UK, and in USA, Maine coon, domestic longhair, Russian Blue and Siamese. Norwegian forest cats are at increased risk in Scandinavia.

In people, the most common forms of diabetes are type 1 (resulting from immune mediated β-cell destruction causing absolute insulin deficiency) and type 2 (characterized by insulin resistance with concomitant β–cell dysfunction). The majority of neutered diabetic dogs have an absolute insulin deficiency that is in some ways similar to type 1 diabetes in people, but this form appears to be very rare in cats. The etiology of diabetes in dogs is multifactorial, and likely involves genetic factors, poorly understood environmental factors, and/or diseases of the exocrine pancreas (eg. pancreatitis), which trigger β-cell injury and inflammation. As in humans, there is a seasonal influence with the incidence peaking in winter. Other factors such as chronic insulin resistance secondary to glucocorticoid administration or obesity, and diseases which antagonize insulin's actions would be expected to hasten onset of clinical signs when superimposed on a reduced capacity to secrete insulin as a result of immune-mediated β-cell destruction.

Diabetes in cats is similar to Type 2 diabetes in people. Insulin resistance is multifactorial and associated with genetic factors, obesity, physical inactivity, male gender, and glucocorticoid steroids.