In the present review, we have tried to briefly define the types of hypoglycemia which may be seen most frequently in medicine. Wherever possible, mechanisms and appropriate diagnostic tests have been described. Treatment of hypoglycemia has been purposely avoided, since therapy should be obvious once diagnosis is made and mechanisms are understood. While it is recognized that this review is not exhaustive, it should cover over 95 per cent of the cases of hypoglycemia encountered by the practicing physician.