Glycogen is the principal storage form of glucose in animal cells. In humans, the most glycogen is found in the liver (10% of the liver mass), whereas muscles only contain a relatively low amount of glycogen (1% of the muscle mass). In addition, small amounts of glycogen are found in certain glial cells in the brain.

Sometimes called "animal starch" for its resemblance with starch found in plants, it is stored in liver and muscle cells and can be converted to glucose if needed. In the liver this conversion is regulated by the hormone glucagon. Under certain conditions, between meals for instance, liver glycogen is an important source of blood glucose. Muscle cell glycogen appears to be only for local use. Glycogen is the primary glucose (energy) storage mechanism. It is stored in the form of granules in the cytosol which is where glycolysis takes place. These granules contain both glycogen and the necessary enzymes for its conversion into glucose.

Glycogen is a highly branched glucose polymer. It is formed of small chains of 8 to 12 glucose molecules linked together with &alpha (14) bonds. These small chains are in turn linked together with &alpha (16) bonds. A single molecule of glycogen can be made of up to 120,000 molecules of glucose. It is generated from glucose by the enzyme glycogen synthase. This process is called glycogenesis. The addition of a glucose molecule to glycogen takes two high energy bonds: one from ATP and one from UTP. Its breakdown into glucose, called glycogenolysis, is mediated by the enzyme glycogen phosphorylase. It's highly branched.

Glycogen is a quick storage vehicle for the body to keep large amounts of glucose when it is not needed by the body. It is classed as a polysaccharide. Although much like amylopectin, glycogen contains more branched chains and has a higher molecular weight. It is stored in both the liver and muscles, but the liver store is more readily available for energy and blood sugar level maintenance, while the muscle store is mostly used for muscle fuel.

Glycogen stores of readily available glucose to supply the tissues with an oxidizable energy source are found principally in the liver, as glycogen. A second major source of stored glucose is the glycogen of skeletal muscle. However, muscle glycogen is not generally available to other tissues, because muscle lacks the enzyme glucose-6-phosphatase.

The major site of daily glucose consumption (75%) is the brain via aerobic pathways. Most of the remainder is utilized by erythrocytes, skeletal muscle, and heart muscle. The body obtains glucose either directly from the diet or from amino acids and lactate via gluconeogenesis. Glucose obtained from these two primary sources either remains soluble in the body fluids or is stored in a polymeric form, glycogen. Glycogen is considered the principal storage form of glucose and is found mainly in liver and muscle, with kidney and intestines adding minor storage sites. With up to 10% of its weight as glycogen, the liver has the highest specific content of any body tissue. Muscle has a much lower amount of glycogen per unit mass of tissue, but since the total mass of muscle is so much greater than that of liver, total glycogen stored in muscle is about twice that of liver. Stores of glycogen in the liver are considered the main buffer of blood glucose levels.