The subject of muscle diseases is a very wide and intricate one, and here we shall deal only with biochemical research on two forms – progressive muscular dystrophy and glycogen storage disease. This chapter came to be written because the muscle biochemist is so often confronted with the question ‘Does all this research on muscle help in the cure of muscle diseases?’ The short answer at present is ‘No’ as far as these two types are concerned. Both are hereditary, and in the case of glycogen storage disease the defect has been traced in different types to lack of a particular enzyme. In the case of progressive muscular dystrophy it has been considered clear that the primary cause is in the muscle itself and not in any deterioration of its nerve supply, but in spite of much experimentation this cause has not been found. Nevertheless much ground has been cleared, and we can say a little about the various views held as to possible causes. It must, however, always be remembered in what follows that the work of the last few years on the effects of cross-innervation on the enzyme patterns of red and white muscles suggests that the nerve supply exerts a more subtle influence on muscle metabolism than had previously been realised. The possibility thus remains that in muscular dystrophy some specific change occurs in the nerve without visible degeneration.