Hypoglycemia is a clinical and biological syndrome, caused by an abnormal decrease in plasma glucose levels to below 0.55 g/l (3.0 mmol/l). Hypoglycemia is responsible for non-specific signs and symptoms which should be noted in a particular pathological context, and for secretion of counterregulatory hormones (mainly glucagon and catecholamines). Difficulty in identifying the etiology is variable, based upon history and physical examination, and hormonal investigations or imaging procedures, according to the results. Drug-related hypoglycemia is the most frequent observed cause (mainly in insulin-treated diabetic patients, but many drugs may be involved), followed par toxicity (alcohol mainly). Tumor-induced hypoglycemia is secondary to inappropriate insulin secretion by a beta-cell pancreatic tumor (insulinoma), and, rarely to an extrapancreatic mesenchymal large tumor secreting IGF-II. Hypoglycemia is present in other diseases, such as hormonal deficiencies, hepatic, or renal failure, or acute cardiac insufficiency. Multifactorial hypoglycemia seems to be underdiagnosed, mainly in hospitalized, underfed older patients with severe disease or sepsis. Autoimmune hypoglycemia is rare, due to insulin or insulin-receptor autoantibodies. Reactive hypoglycemia is observed after gastrectomy, but true primitive hypoglycemia appears to be rare, with false excess diagnosis in the majority of the cases.