The history, technique, complications, and evaluation of myocardial biopsy, particularly in cardiomyopathies, have been reviewed and discussed. As expected, specific histological abnormalities for the diagnosis of cardiomyopathies were hard to obtain in such small fragments of myocardium. Rather, myocardial biopsy at present provides information on the severity and prognosis of cardiomyopathies, especially those with a diffuse disease process. However, morphologic examination is definitely valuable in diagnosing cardiomyopathies, even if it merely confirms the clinical diagnosis, because none of the information presently given by any one diagnostic tool is specific enough to correctly diagnose cardiomyopathy. Endomyocardial biopsy is most useful from a diagnostic standpoint in acute cardiac rejection in transplant recipients as well as in secondary myocardial diseases: myocarditis, amyloidosis, hemosiderosis, glycogen storage disease, sarcoidosis, etc. Advances in biochemical, immunologic, and more specific morphologic analyses will increase the usefulness of myocardial biopsy in diagnosing and assessing the etiology of idiopathic cardiomyopathy.