Hypoglycemia (hypoglycæmia in the UK) is a medical term referring to a pathologic state produced by a lower than normal amount of sugar (glucose) in the blood. The term hypoglycemia literally means "low blood sugar". Hypoglycemia can produce a variety of symptoms and effects but the principal problems arise from an inadequate supply of glucose as fuel to the brain, resulting in impairment of function (neuroglycopenia). Derangements of function can range from vaguely "feeling bad" to coma, and (rarely) permanent brain damage or death. Hypoglycemia can arise from many causes, and can occur at any age. The most common forms of moderate and severe hypoglycemia occur as a complication of treatment of diabetes mellitus with insulin or oral medications.

Endocrinologists (specialists in disorders of blood glucose metabolism) typically consider the following criteria (referred to as Whipple's triad) as proving that an individual's symptoms can be attributed to hypoglycemia:

1. Symptoms known to be caused by hypoglycemia
2. Low glucose at the time the symptoms occur
3. Reversal or improvement of symptoms or problems when the glucose is restored to normal

However, not everyone has accepted these suggested diagnostic criteria, and even the level of glucose low enough to define hypoglycemia has been a source of controversy in several contexts. For many purposes, plasma glucose levels below 70 mg/dl or 3.9 mmol/L are considered hypoglycemic, but these issues are elaborated in more detail below.