Nonhuman primates have been used for a variety of studies on diabetes mellitus. Spontaneous, natural forms of diabetes have been well documented in several species; there are limited data on numerous other species that indicate diabetes or a diabetes-like syndrome. The causes and manifestations of spontaneous diabetes, their prevalence, and their severity vary among species. Diabetes has also been induced in nonhuman primates with streptozotocin, alloxan, hypothalamic lesions, or pancreatectomy. The extent and severity of metabolic and hormonal abnormalities vary according to the method of induction, the individual monkey, and the species. Metabolic, hormonal, and pathologic abnormalities present in human diabetics also occur in monkeys with either spontaneous or induced diabetes. Hyperglycemia and impaired glucose clearance are common, lipid concentrations are elevated, and hemoglobin A1c concentrations are increased in hyperglycemic monkeys. Monkeys may have fasting hypo- or hyperinsulinemia; insulin responses are often impaired in glucose tolerance tests. Glucagon concentrations may be increased. Aortic atherosclerosis, muscle capillary microangiopathies, cataracts, and glomerulosclerosis have been documented. Primate size and longevity allow longitudinal studies with procedures that may not be feasible in smaller animals or in human beings. Nonhuman primates may be the models of choice for studies on selected aspects of diabetes and its secondary complications.