This review describes clinical, biochemical and genetic features of the four inborn errors affecting muscle glycogen breakdown, namely deficiencies of phosphorylase, phosphorylase kinase, amylo-1,6-glucosidase and acid alpha-glucosidase. They are characterized by a wide spectrum of clinical manifestation, affecting age of onset, clinical features, progress of disease and tissue involvement. Biochemically, variability of all four enzyme deficiencies is evident in terms of differences in residual enzyme present in tissues, and in the presence or absence of enzyme protein. Genetic heterogeneity, which has been documented in each of the enzyme deficiencies, manifests itself in terms of the presence, absence, quantity or size of mRNA. In phosphorylase deficiency heterogeneity has also been documented at the DNA level. In acid maltase deficiency nine mutant phenotypes have been described affecting various stages of lysosomal enzyme processing.