Signal-averaged electrocardiography (ECG) is a new noninvasive test for identifying patients at risk for ventricular arrhythmias. This computerized method of analyzing standard ECGs identifies particular microvolt-level signals called late potentials. Late potentials have been correlated with clinical ventricular tachycardia, are predictive of ventricular tachycardia inducibility at the time of electrophysiologic testing, and are predictive of arrhythmic events after myocardial infarction. In this review, we describe late potentials, the method of obtaining and processing the signal-averaged ECG, and clinical studies in various patient groups that have assessed the predictive value of the signal-averaged ECG for identification of patients at risk for subsequent ventricular arrhythmias.