Most supraventricular tachycardias can be diagnosed confidently using the standard 12-lead electrocardiogram, the more so if such a tracing in sinus rhythm is also available. The underlying mechanisms may often be determined and these may have important therapeutic consequences. This chapter reviews the electrocardiographic appearances of the supraventricular arrhythmias. Extrasystoles are common but rarely require specific treatment. Automatic atrial tachycardias are common and usually amenable to simple management. Reentrant circuits related to atrial and atrioventricular tachycardias are also described, and where possible, treatment is related to underlying mechanisms.