Glycogen is a crucial source of energy in the initial stages of muscle activity and during exercise of high intensity. There are 10 well-defined biochemical defects of glycogen metabolism expressed in muscle and affecting the following enzymes: alpha 1,4 glucosidase (glycogenesis type II), debrancher enzyme (III), brancher enzyme (IV), phosphorylase (V), phosphofructokinase (VII), phosphorylase b kinase (VIII), phosphoglycerate kinase (IX), phosphoglycerate mutase (X), lactate dehydrogenase (XI). These disorders cause two main syndromes: one characterized by exercise intolerance with cramps and myoglobinuria, the other by fixed weakness. However, there are examples of clinical and biochemical heterogeneity for each disease, and molecular genetic analysis is already showing evidence of genetic heterogeneity. Although our understanding of the biochemical errors has progressed considerably, the pathogenesis of symptoms and signs remains incomplete.