Endomyocardial biopsy is an accepted, useful invasive tool for the analysis of human endomyocardium at the cellular and subcellular levels. It is applicable in the evaluation of specific diseases including cardiac allograft rejection, myocarditis, anthracycline cardiotoxicity, and infiltrative cardiomyopathies. The procedure can be performed in a cardiac catheterization room on an outpatient basis. The technique is quite safe when performed by trained cardiologists. Left ventricular biopsies are also safe but require systemic heparinization to prevent thromboembolization. The clinical indications for performing an endomyocardial biopsy include routine followup and suspected rejection of cardiac allograft, suspected myocarditis, monitoring or diagnosis of suspected anthracycline cardiotoxicity, and suspected secondary cardiomyopathies. Left ventricular endomyocardial biopsy is indicated for diseases that predominantly involve the left side of the heart, including left heart irradiation, cardiac fibroelastosis in infants, endomyocardial fibrosis, and scleroderma heart disease, and when right ventricular biopsy is unsuccessful. Endomyocardial biopsy is increasingly being used for research in the areas of tissue biochemistry, primary and valvular cardiomyopathies, immunology, beta receptor enzymology, drug interactions, and myocardial fibrosis. Endomyocardial biopsy has not been shown to be clinically useful in the evaluation of primary, dilated, hypertrophic, or alcoholic cardiomyopathies. These disease processes all lack pathognomomic microscopic abnormalities, and subclassification has neither been successful nor therapeutically useful. In addition, this technique is limited in diagnosing any cardiac abnormality that is not diffuse, inasmuch as only a few samples of the endomyocardial layer are obtained for evaluation. Therefore, a negative biopsy result is not 100 percent specific in excluding certain diseases. A further limitation of this technique is the need for an experienced cardiac pathologist who is well versed in interpretation of biopsy specimens. Finally, there should be a sufficiently large case load to train and to maintain skilled practitioners so that the procedure can be performed with little risk. The role of endomyocardial biopsy will continue to expand as research continues to find more uses for the technique and as more clinicians become skilled in its use.