Currently, there are two main categories of cardiac valve replacements: mechanical models, or bioprostheses. Although all available models provide comparable hemodynamic benefitsin vivo,none is perfect. Replacement valves must be selected on an individual basis, taking into account the patient's heart size and severity of valve damage; age; medical history; activity level and lifestyle; and postoperative quality of life. Bioprostheses are in general superior to mechanical substitutes. They close more rapidly, have central flow, are quieter, and are less prone to thromboembolic complications. Their durability, however, remains a significant concern. Reliability of tissue valves will likely be enhanced through superior engineering study and analysis of the effects of various tissue treatments and preservation; intensive research in the field of polymer science to develop a sufficiently strong man-made leaflet membrane; and development of new geometrical valve configurations which minimize mechanical stress.