Once thought to be solely a disease of insulin deficiency, diabetes mellitus now is recognized as a disorder with multiple pathogenetic mechanisms. Newer terminology identifies those uncommon patients with true insulin deficiency as having insulin-dependent diabetes (IDDM), while the majority of patients with diabetes have some residual insulin secretion but may have a disorder of insulin receptor number or affinity. These patients have non-insulin dependent diabetes (NIDDM). Other patients may have gestational diabetes, impaired glucose tolerance, a potential for glucose intolerance, or a previous history of diabetes. A few patients will have diabetes secondary to a known cause, such as pancreatitis or Cushing's syndrome. Understanding this nosological approach to diabetes should enhance the clinician's decisions regarding therapy.