Invasive electrophysiological testing has contributed importantly to the objective evaluation and management of patients at high risk for sudden cardiac death. The clinical application of the technique is based on the hypothesis that the reproducible induction of ventricular arrhythmias by programmed cardiac stimulation constitutes a marker of risk for spontaneous ventricular arrhythmias and sudden death as well as an objective end point to guide the selection of antiarrhythmic therapy. The value of electrophysiological testing is well established in patients with ischemic heart disease and a history of sustained ventricular tachycardia or fibrillation and in some subsets of patients with unexplained syncope. More recently, the technique has been used by some investigators to identify individuals at high risk for sudden death among patients with recent myocardial infarction and those with left ventricular dysfunction and recurrent nonsustained ventricular tachycardia. The predictive value of the technique in patients with nonischemic heart disease is unknown. In addition to its use as an objective end point in the selection of antiarrhythmic drug therapy, invasive electrophysiological testing has advanced our knowledge of the mechanisms of life-threatening ventricular arrhythmias and contributed importantly to the development of new therapies, such as implantable arrhythmia control devices and catheter ablation techniques.