Wide complex tachycardias (WCT) are frequently encountered in the emergency department. Causes of WCT vary from benign (eg, supraventricular with rate-related aberrancy) to life threatening (eg, atrial fibrillation with WPW, or ventricular tachycardia). It is imperative that emergency physicians are familiar with the clinical presentation, underlying causes, and electrocardiographic features of the different causes of WCT. Treatment of WCT depends on patient stability, regularity of the rhythm, and QRS morphology. When in doubt, monomorphic WCT should be presumed to be ventricular tachycardia and treated as such.