Electrophysiologic studies are indicated in patients with sustained paroxysmal ventricular tachycardia, ventricular fibrillation or aborted sudden death. These studies allow determination of mechanism and reproducibility of initiation as well as pacing termination of ventricular tachycardia, against which the effects of pharmacologic or nonpharmacologic therapies can be tested. Such studies are also indicated in certain patients with syncope in whom a strong suspicion exists for an arrhythmic cause. The content and conduct of electrophysiologic testing in these patients require attention to the physiology of the conduction system and systematic programmed stimulation of the right ventricle. The stimulation protocol should include, if necessary, twice-threshold stimulation at 2 sites at 3 or more cycle lengths, with up to 3 extrastimuli. Sufficient variability exists in electrophysiologic testing as in other clinical methods calling for careful attention to the reproducibility of tachycardia induction in a given patient, lest chance alone mimic beneficial or deleterious effects of antiarrhythmic regimens. Mapping-directed surgery for ventricular tachycardia remains the most effective therapy in patients with sustained monomorphic ventricular tachycardia with a mortality similar to other forms of medical therapy.