The identification of the gene responsible for Becker and Duchenne muscular dystrophy and the characterization of the corresponding biochemical defect, dystrophin, has dramatically changed the management of patient and families with these disorders. The majority of patients with a muscle disease are now recognized to have some form of dystrophinopathy, and the correct identification of these patients is critical not only for patient care, but for family genetic counseling. The clinician must now be familiar with an array of molecular diagnostic procedures and their correct implementation and interpretation. The range of clinical disorders caused by dystrophin abnormalities has expanded to include a large range of myopathic conditions, including the presentation of cardiomyopathy. Radically new therapeutic strategies for Duchenne muscular dystrophy are being tried, and many provide hope that treatment may soon be available.