Intensively treated patients with diabetes have a three-fold increased risk of severe hypoglycemic episodes with an attendant four percent mortality. These findings, unfortunately undermine attempts to achieve normoglycemia in diabetic patients. It has been proven that antecedent hypoglycemia is a major factor responsible for blunting metabolic, neuroendocrine and also autonomic responses to subsequent hypoglycemia. Diabetic patients with good glycemic control become unable to recognize symptoms of hypoglycemia. Lack of symptoms, or hypoglycemia unawareness, is part of the syndrome called hypoglycemia associated autonomic failure. This syndrome also includes inadequate neuroendocrine hormonal responses and reduced glycemic thresholds for counterregulatory hormonal secretion. Factors regulating the magnitude of hypoglycemia associated autonomic failure include antecedent duration and frequency of hypoglycemia, prior episodes of exercise, and autonomic neuropathy. Understanding the pathophysiology of this syndrome will provide a foundation for therapy aimed at preventing severe hypoglycemia during intensive metabolic control. This article will review our current understanding of the mechanisms responsible for hypoglycemia associated autonomic failure.