Few if any medical decisions are of more urgent importance than the accurate discrimination between ventricular tachycardia and supraventricular tachycardia with ventricular aberration, and probably no common diagnosis is more often missed. Yet the distinction can often be readily made with a knowledge of the several clues here described. These include QRS morphology, polarity and width; and clinical or electrocardiographic evidence of independent atrial activity. Knowledge and application of these serviceable clues should enable the clinician to establish a correct diagnosis in 90% of wide-QRS tachycardias without resorting to invasive studies.