**Abstract**

Narrow QRS complex tachycardias or Wide QRS complex tachycardias are common problems encountered in clinical practices. Although such tachycardias often occur in patients with a normal anatomy and/or function of heart and rarely represent life-threatening conditions, they are common sources of morbidity and/or mortality. Narrow QRS complex tachycardias are fast cardiac rhythms with QRS duration of 120 ms or less while wide QRS complex tachycardias are fast cardiac rhythms with QRS duration of 120 ms or more. Origins of narrow QRS complex tachycardias are above or within the His bundle. Wide QRS complex tachycardias can be ventricular tachycardias, supra-ventricular tachycardias with bundle branch block or accessory pathway. The purpose of this chapter is to present the differential diagnosis of narrow and wide QRS complex tachycardias.

**Keywords:** narrow QRS tachycardia, wide QRS tachycardia, tachycardia, electrocardiograms, regular rhythm NCTs, irregular rhythm NCTs
