Diabetes mellitus is composed of a heterogeneous group of disorders characterized by high blood glucose levels. Four major types of diabetes have been defined by the National Diabetes Data Group. Insulin-dependent diabetes (IDDM), also called type I diabetes, is characterized by abrupt clinical onset, insulinopenia, proneness to ketosis even in the basal state, and dependence on exogenous insulin to sustain life. Non-insulin-dependent diabetes (NIDDM), also called type II diabetes, may remain relatively asymptomatic for years. Insulin levels may be normal, lower than normal, or elevated as a consequence of insulin resistance. Ketosis is not part of the general clinical picture except in times of metabolic stress, although the classic complications of diabetes can be expected to develop in long-duration diabetics. Gestational diabetes (GDM) refers to the recognition of abnormal glucose intolerance in pregnancy, although unrecognized abnormal tolerance may indeed have predated the pregnancy. Rates of macrosomia are higher than in non-GDM pregnancies, but fetal mortality and congenital anomalies appear to be no greater than in the general population. Other types of diabetes include a number of diverse conditions in which glucose intolerance is a feature and in which it may be etiologically related. Impaired glucose tolerance (IGT) is a class that encompasses persons whose glucose tolerance is intermediate between normal and diabetic. These individuals do not manifest the microvascular complications of diabetes, but they appear to have higher rates of macrovascular disease associated with the known cardiovascular risk factors. Two statistical risk categories have also been defined that replace the older terms prediabetes, potential diabetes, and latent diabetes. Diabetes can be diagnosed by the presence of classical signs and symptoms of diabetes and unequivocally elevated blood glucose levels; by a fasting plasma glucose greater than or equal to 140 mg/dl; or by an abnormal oral glucose tolerance test, with a venous plasma glucose value greater than or equal to 200 mg/dl at 2 hours after 75 grams oral glucose, being a hallmark criterion for diabetes. For the latter two criteria, the abnormality should be reconfirmed at a later occasion before a definitive diagnosis of diabetes is made. The oral glucose tolerance test has been standardized at a 75-gram glucose (or carbohydrate equivalent) load, given in the morning after an overnight fast. Glucose should be determined for two hours after administration of the challenge.