Glycogen storage diseases (GSD) are caused by enzyme defects of glycogen degradation. Some enzyme defects cause glycogen storage in the liver due to the fact that the deficient enzyme is mainly localized in the liver. Some enzyme defects are localized in muscles, and some are more generalized. The classification is presented in Table 1, which shows that GSD are not only defined by the deficient enzyme but also by a type number, which reflects the historical sequence of elucidation. In this chapter the GSD are not described according to that chronological order, but rather according to the clinical presentation, mainly hepatic, generalized, or mainly myogenic.