Over the last two decades, the implantable artificial heart has evolved from an idea to a device capable of completely supporting the circulation for periods now exceeding 5 months. Although initial animal studies were limited by thromboembolism and device breakage, the usual causes of death in experimental animals are now infection, atrioventricular valve obstruction, elastomer bladder calcification, or inadequate cardiac output because of the relatively rapid growth of the young calves. As a result of the bulky nature of the energy converter and the substantial risk of infection with large diameter percutaneous tubes, clinical use of their air-powered artificial hearts will be limited to patients who are awaiting or being prepared for heart transplantation. Artificial hearts with implanted energy converters are being developed for permanent heart replacement. These devices require well-designed, durable mechanical components and sophisticated control systems. Although initial designs centered around thermal engines powered by a completely implantable nuclear energy source, the excessive cost and potential dangers have shifted the focus away from the nuclear system. Several electrically driven artificial hearts, based on samarium-cobalt magnet brushless direct-current motors, are now undergoing bench testing and will be ready for long-term animal studies within 2 years. This research will culminate with the availability of an "off-the-shelf" electrically powered artificial heart for use in patients with a wide range of nonrepairable forms of end-stage heart disease.