Summary In conclusion, the glycogen storage diseases represent a clinically heterogeneous group of disorders that usually become apparent in early infancy and reflect the consequences of a deficiency of enzymes essential for the normal synthesis and degradation of glycogen. The symptomatology depends primarily on the organs that are involved in the enzymatic defect. Most diseases with primarily hepatic involvement manifest abnormalities in glucose homeostasis, whereas those types with enzyme deficiency in muscle present with muscle weakness, cramps and frequently myoglobinuria. In the former group, most of the physical findings and metabolic alterations can be explained on the basis of the primary defect in maintenance of a normal blood glucose. Most of these diseases appear to be inherited as autosomal recessives and many of them become less severe as the child gets older. Although the specific type of glycogen storage disease may be indicated by the clinical presentation and the results of certain functional tests, conclusive diagnosis can be established only by enzyme assay of the involved tissue. No specific therapy exists at present, but supportive measures have been very beneficial in some of the varieties of glycogen storage diseases, whereas other types have been uniformly fatal despite all therapeutic attempts. The study of these diseases has played an important role in elucidating the metabolic pathways involved in glycogen synthesis and degradation, as well as the interrelationships of carbohydrate, fat and protein metabolism.