Left ventricular assist devices are being used increasingly more often in patients with medically refractory heart failure. The HeartMate (Thermo Cardosystems Inc., Waltham, MA) and Novacor (Baxter Healthcare Corp., Deerfield, IL) assist devices are fully implantable devices that permit patients to be completely ambulatory. Presently, their chief application is as a bridge to cardiac transplantation, but the possibility that these devices could be used as long-term therapy in nontransplant patients is being investigated. The physiology of these devices, the complications of their long-term use, and the possibility of myocardial recovery after prolonged profound ventricular unloading are reviewed in this article. Finally, the exercise performance of patients using these devices is compared with that of patients with severe heart failure.