Hereditary myopathies have been well defined on a molecular basis in recent years as a consequence of concerted efforts to identify the causal genetic mutations. This has not only clarified heretofore uncertain clinical and pathologic aspects such as variations in severity but also led to markedly enhanced diagnostic capabilities and also novel molecular therapeutic strategies. The most impressive, significant discoveries relate to the commonest severe disorders, namely Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA) syndromes. DMD, an X-linked disorder, is attributable to pathologic variants in the dystrophin gene, a huge segment of DNA at the p21 region on the short arm of the X chromosome; its ~2500 kb size makes dystrophin unusually susceptible to a high mutation rate. Newborn screening, based on detecting high levels of blood creatine kinase followed by DNA analyses facilitates early diagnosis and institution of helpful but not yet curative therapies. Infantile SMA is an autosomal recessive disorder due to mutations in two genes at the 5q11-q13 locus that encodes a neuronal apoptosis inhibitor protein and the survival motor neuron gene, respectively. Most cases are due to a homozygous deletion in the SMN1 gene of 5q13. This discovery permits application of a newborn screening test with complex DNA analysis, but improved therapies are needed before this strategy should become routine. Other myopathies such as myasthenia gravis and a variety of congenital neuropathies have also been better characterized through molecular research and thus can be diagnosed and managed more effectively.