Hypoglycaemia or hypoglycemia is the medical term for a pathologic state produced by a lower than normal level of blood glucose. The term hypoglycemia literally means "under-sweet blood" (Gr. hypo-, glykys, haima).

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, seizures, and (rarely) permanent brain damage or death. Hypoglycemia can arise from many causes and can occur at any age. It also sometimes occurs at random.

The most common forms of moderate and severe hypoglycemia occur as a complication of treatment of diabetes mellitus treated with insulin or less frequently with certain oral medications. Hypoglycemia is usually treated by the ingestion or administration of dextrose, or foods quickly digestible to glucose.

Endocrinologists (specialists in hormones, including those which regulate glucose metabolism) typically consider the following criteria (referred to as Whipple's triad) as proving that 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; these issues are detailed below.