Narrow complex tachycardias are those cardiac rhythms with a ventricular rate of more than 100 beats per minute and a QRS complex width of less than 0.12 seconds. They originate either from the SA node, from atrial tissue itself, or from in or around the AV node. The term SVT is generally accurate for such tachycardias. By diagnosing the source of an SVT and appreciating its likely cause, therapy can be more precisely, safely, and effectively guided to treat these patients. Atrial and junctional rhythms can be treated with vagal maneuvers, drugs from classes I to IV and other antiarrythmic agents, magnesium, and cardioversion. Some patients may be candidates for surgical or catheter ablation.