Ventricular fibrillation, a loss of synchronus electrical activity in the heart which leads to hemodynamic collapse, is a leading cause of death. Because of the devastating personal and societal effects of this phenomenon, the automatic cardioverter-defibrillator has been developed for automatic detection and termination of the arrhythmia and is in widespread clinical use. Advances in circuits, leads, wave-forms, and signal processing along with increased knowledge of the mechanisms of fibrillation have led to continuing improvements in this device, extending its use to many patients. A device has also been developed for the automatic or semiautomatic treatment of atrial fibrillation, an arrhythmia less life-threatening than ventricular fibrillation, but still a serious health problem. Continued improvement of these devices and the development of qualitatively new approaches hold great promise for exciting therapeutic advances in this area. Copyright © 1999 by Annual Reviews. All rights reserved.