The glycogen storage diseases of muscle are the only inherited diseases of muscle in which the biochemical abnormalities are known. Despite impressive advances in knowledge during the past decade there are vital gaps in understanding. In none of these diseases is treatment satisfactory. In none of these disorders can theory relate the symptons of the disease (weakness, cramps, myoglobinuria) to the enzymatic defect. In several there are biochemical abnormalties that do not permit explanation in terms of the enzymatic defect. Individual patients and families do not fit into simple schemes of genetic and biochemical analysis. Different proteins have the same enzymatic activities in different organs, apparently under separate genetic control. One enzyme in particular, acid maltase, plays an uncertain role in the normal metabolism of glycogen; lack of this enzyme in the infantile Pompe's disease and myopathies of later onset are of uncertain significance. For these reasons, and others, there is much to be learned about these diseases wer know best. About more common diseases like the muscular dystrophies, we know even less. It is one thing to deliver medical care and something else to have medical care to deliver.