Hypoglycemia is a topic that has generated a great deal of confusion. Although the word means “low blood glucose,” the diagnosis of hypoglycemia is controversial, because it is sometimes defined strictly on the basis of an arbitrary blood glucose level (chemical hypoglycemia), sometimes in terms of symptoms (clinical hypoglycemia), and sometimes as a combination of glucose level and symptoms. The most readily accepted aspect of hypoglycemia is division into two clinical categories: one in which symptoms occur after fasting (fasting hypoglycemia) and one in which symptoms occur after eating (postprandial hypoglycemia). If the blood glucose level drops rapidly, symptoms tend to be similar to those associated with release of epinephrine (adrenergic) and include anxiety, sweating, palpitation, tremor, and hunger. If hypoglycemia persists, CNS glucose deprivation occurs (neuroglycopenia) and symptoms resemble those of cerebral hypoxia, such as lethargy, headache, confusion, bizarre behavior, visual disturbances, syncope, convulsions, and coma. Symptoms associated with fasting hypoglycemia tend to be one or more of those associated with CNS neuroglycopenia, and those associated with postprandial hypoglycemia tend to be adrenergic. However, there is some overlap in symptoms between the two groups. In general, symptoms due to fasting hypoglycemia have a much higher incidence of visual disturbances, bizarre behavior or personality changes, convulsions, and loss of consciousness, especially prolonged loss of consciousness. Postprandial hypoglycemia tends to be more abrupt in onset and is usually self-limited.