The indications for clinical electrophysiologic study have continued to evolve since the introduction of this technique in the mid 1960s. They now include evaluation of sinus node dysfunction, AV conduction disturbances, syncope of unknown etiology, supraventricular arrhythmias with and without Wolff-Parkinson-White syndrome, sustained ventricular tachycardia, and out-of-hospital cardiac arrest. The rate of complications for this procedure is acceptable, with the potential benefits being accurate diagnosis and prescription of therapy.