These heart muscle diseases, associated with systolic or diastolic dysfunction, are associated with a high prevalence and morbidity, because of the frequent evolution towards congestive heart failure or sudden death. Dilated cardiomyopathy is the first cause of heart transplantation and Hypertrophic cardiomyopathy is the first cause of sudden death in athletes. Familial and monogenic forms of these diseases are frequent and this should lead to a systematic cardiological familial inquest in first degree relatives of a patient. Although they were previously called as "idiopathic", the pathophysiology of the disease is actually under investigation thanks to molecular genetics. This completely new knowledge should lead to the development of new therapeutic strategies.