Spontaneous diabetes is a common occurrence in many animal species. In addition, animals can be rendered diabetic by a wide variety of experimental procedures. Diabetic animals may be regarded as models of the disease in man. However, such animals display a wide diversity of pathophysiology, and, in fact, no animal syndrome corresponds precisely to any type of diabetes in human subjects. The most common diabetes syndromes in animals occur in the context of obesity, hyperinsulinemia and insulin resistance. Many such syndromes remit spontaneously. Dietary restriction and weight reduction effectively reverse some of these syndromes, but in other cases only partial correction of the syndrome occurs. Diabetes in lean animals is less common. The diabetes of lean animals is more frequently characterized by hypoinsulinemia, ketosis and insulin dependence than is the case with obese animals. Diabetes may be produced experimentally by means of surgery, viral infection or the administration of various hormones and chemical agents. Both the spontaneous and experimental animal models have been used effectively to study the etiologies, complications, treatments and prevention of diabetes.