Significant advances in the basic science of implantable cardioverter- defibrillator therapy have led to improvements in defibrillation waveforms and in capacitor technology. Nonthoracotomy devices with biphasic waveforms can be implanted with a near 100% success rate. Although fewer patients with implantable cardioverter-defibrillators are being treated with concomitant antiarrhythmic drug therapy, sotalol appears to decrease the defibrillation threshold. Controversy still exists over the optimal design for defibrillator sensing leads. Tachyarrhythmia detection enhancements increase specificity for sensing ventricular tachycardia but may risk undersensing. A variety of subsets of patients receiving implantable cardioverter-defibrillators have been identified; patients presenting with ventricular fibrillation appear to have the most unfavorable prognosis. Controversy exists as to the true impact of implantable cardioverter-defibrillator therapy on subsequent survival. Randomized clinical trials such as AVID (Antiarrhythmics Versus Implantable Defibrillators) are designed to determine the true benefits of implantable cardioverter- defibrillators and may lead to expanded indications.