Electrical therapy for tachyarrhythmias attempts to achieve one or more of three aims: a) prevention of tachycardia; (b) control of the hemodynamic effect of tachycardia; (c) termination of tachycardia. In practice, long term control of tachycardia in selected patients can be achieved with implantable devices which can automatically recognize and terminate tachycardias. Termination can be achieved with a number of pacing modalities. These pacing modalities are reviewed in this article and some guidelines to the choice of modality are given. Patients with supraventricular tachycardia are often more appropriately treated with drugs or surgery but some can be effectively treated with antitachycardia pacing. Some patients with ventricular tachycardia can be successfully treated with these devices but this group is at risk of tachycardia acceleration or degeneration in response to pacing. An implantable cardioverter-defibrillator should be used as a backup in these patients. Present generation devices now incorporate antitachycardia pacing, low energy cardioversion, and higher energy defibrillation in the same unit.