Glycogen is the storage form of glucose in animal cells. Its degradation can rapidly provide fuel for energy production (particularly important in muscle), or replenish blood glucose during fasting by the liver. Genetic defects of glycogen metabolism give rise to glycogen storage diseases (GSDs), manifesting histologically in abnormal quantity or quality of glycogen in the cells. GSDs can be caused by defects of proteins participating in the synthesis or degradation of glycogen itself, in the glycolytic degradation of glucose phosphates in muscle and erythrocytes, in the release of glucose from liver and kidney into the bloodstream, in the clearance of glycogen from lysosomes (all, "primary GSDs"), or in the control of these pathways ("secondary GSDs"). Most genes responsible for classical, primary GSDs have probably been identified, and future progress in understanding the biochemical and genetic defects underlying unsolved disorders presenting with glycogen storage abnormalities will perhaps be predominantly in the field of secondary GSDs.