Diabetes mellitus is one of the most common endocrinopathy in cats. It is due to lack of insulin. There are two types of insulin-dependent diabetes (IDDM), extremely rare in cats, and insulin-independent (NIDDM). The chemical markers of diabetes are chemical compounds of the blood and urine. The final diagnosis of diabetes in cats is based on clinical signs of the disease and results of laboratory blood and urine tests. Cats are often hyperglycemia caused by stress. In this case urine does not contain increased glucose concentration, which makes it easier to distinguish between diabetes and hyperglycemia caused by stress. In diabetes, in addition to increasing the amount of glucose in blood serum and urine, basic clinical symptoms are present: polyuria, polydipsia, polyphagia, loss of body weight. The main markers of diabetes mellitus are: glucose (blood, urine), ketones (urine), HbA1c, fructosamine. Diagnosis is surely established on the basis of clinical symptoms and results of laboratory blood and urine tests. Treatment is more complicated than in dogs. It is known that cats with diabetes are unpredictable in response to exogenous insulin. No type of insulin is routinely effective in maintaining glycemic control, even when administered twice daily, it is not possible to predict which type of insulin will best affect certain cats. The average time of survival of dogs and cats is three years from the diagnosis, but in some animals survival is longer than seven years.