Broad complex tachycardias occur by various mechanisms and may be ventricular or supraventricular in origin. In the emergency setting most broad complex tachycardias have a ventricular origin. However, an arrhythmia arising from the atria or the atrioventricular junction will produce a broad complex if associated with ventricular pre-excitation or bundle branch block. The causes of ventricular and supraventricular tachycardias are generally quite different, with widely differing prognoses. Most importantly, the treatment of a broad complex tachycardia depends on the origin of the tachycardia. This article describes monomorphic ventricular tachycardias; other ventricular tachycardias and supraventricular tachycardias will be described in the next article.