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Glycogen is a complex carbohydrate and the primary form of glucose storage in animals, fungi, and bacteria. It is a branched polymer of glucose units linked together by alpha glycosidic bonds. In vertebrates, glycogen is stored mainly in the liver and muscles. Liver glycogen serves as a readily available source of glucose for the entire body, maintaining blood glucose levels between meals. Muscle glycogen, on the other hand, is primarily used as an energy reserve for muscle contraction during physical activity.

The synthesis of glycogen, known as glycogenesis, is a metabolic pathway that occurs when glucose levels are high, such as after a meal. Conversely, the breakdown of glycogen into glucose, called glycogenolysis, is stimulated when glucose levels are low, providing energy to cells. The regulation of glycogen metabolism is tightly controlled by hormones like insulin and glucagon, which respond to changes in blood glucose concentration. Disorders of glycogen metabolism, known as glycogen storage diseases, can arise from genetic defects in the enzymes involved in glycogen synthesis or breakdown, leading to various health problems.