Diabetes mellitus is defined as a disorder of persistent hyperglycaemia regardless of underlying cause. The typical clinical signs are polydipsia/polyuria and weight loss. The majority of cats have a disorder similar to type 2 diabetes mellitus in humans. Risk factors include obesity, physical inactivity, confinement indoors and prior administration of glucocorticoids or progestogens. The chapter looks at aetiopathogenesis, clinical features, diagnosis, treatment and outcome and prognosis.