We firmly believe that a systematic approach to the 12-lead ECG can provide information that can diagnose the difference between ventricular and supraventricular tachycardia, and in many instances diagnose the mechanism and site of origin of the supraventricular tachycardia. The mechanism of supraventricular tachycardia is able to be diagnosed in more than 80% of narrow complex tachycardias, and one should be able to distinguish supraventricular from ventricular tachycardia in more than 90% of tachycardias. In the presence of aberration, the mechanism of supraventricular tachycardias is more difficult to define unless retrograde P waves are seen. In such instances the morphology of the P wave (if seen), the effect of oscillation of cycle length on the RP interval, and response to vagal maneuvers may be useful in distinguishing mechanisms for ventricular tachycardia.