Hypoglycemia is diagnosed convincingly when typical symptoms are associated with a low plasma glucose concentration and are relieved by glucose administration. It requires urgent treatment (usually with intravenous glucose in the hospital setting), diagnostic explanation, and long-term prevention. The latter is based upon an understanding of the pathogenesis of hypoglycemia in the affected patient. Postabsorptive (fasting) hypoglycemia is often caused by drugs (especially insulin, a sulfonylurea, or alcohol); it can also result from endogenous hyperinsulinism (insulinoma, autoimmune hypoglycemia), a non-β-cell tumor, hormonal deficiencies, or a variety of clinical syndromes including sepsis, cardiac, renal, and hepatic failure, and even inanition per se. Hypoglycemia is a treatable cause of acute morbidity. It is sometimes a cause of chronic morbidity and even mortality that could have been prevented.