The muscular dystrophies are a clinically and genetically heterogeneous group of skeletal muscle-wasting diseases that differ widely in their frequency and pattern of cardiac involvement. Myocardial disease manifesting predominantly as cardiomyopathy and congestive heart failure is characteristic of Duchenne and Becker muscular dystrophies and X-linked dilated cardiomyopathy, whereas conduction system abnormalities that cause heart block, arrhythmias, and sudden death are more commonly seen in limb-girdle type 1B, myotonic, and Emery-Dreifuss muscular dystrophies. Primary defects in the mechanical stabilization of the plasma membrane and signal transduction may underlie these two groups of muscular dystrophies. The identification of several new disease genes has yielded additional insights into the pathophysiology of muscular dystrophy. Molecular genetic and biochemical analyses of patient samples now permit accurate diagnosis and genotype-phenotype correlations. Ultimately, this knowledge will provide the foundation for etiology-specific gene therapy.