Cardiac electrophysiology studies use intracardiac recording and programmed stimulation to define the mechanisms and most appropriate therapy for supraventricular and ventricular arrhythmias. Using these techniques, the majority of clinical tachycardias can be reproducibly initiated and terminated in the electrophysiology laboratory, thereby allowing the most appropriate therapy to be selected. With this approach, antiarrhythmic agents can be tested in a systematic, serialized fashion for efficacy, safety and patient tolerance. With both supraventricular and ventricular tachycardias, suppression of arrhythmia induction predicts freedom from recurrence, whereas inducibility carries a poor prognosis in clinical follow-up. Electrophysiology studies provide a safe and effective approach to the treatment of selected patients with cardiac arrhythmias.