Wide QRS tachycardia is a diagnostic challenge when confronted on a 12-lead electrocardiogram. The differential diagnosis includes: ventricular tachycardia; supraventricular tachycardia with aberration; and Wolff-Parkinson-White syndrome. Confronted with a wide QRS tachycardia, one must determine whether the origin is ventricular or supraventricular because the therapy will differ. The electrocardiographic findings of capture beats, fusion beats and atrioventricular dissociation are highly specific for ventricular tachycardia but not very sensitive. After careful assessment of the 12-lead electrocardiogram following selected diagnostic features, the correct diagnosis of the cause of wide QRS tachycardia can be made in about 90 percent of patients. This article contains a brief discussion of the diagnostic features of wide QRS tachycardia.