Paroxysmal supraventricular tachycardia (PSVT) is a distinct clinical syndrome. Most patients present with the abrupt onset of palpitations, dizziness, dyspnea, or chest pain. The electrocardiogram (ECG) demonstrates a fast heart rate (150-250 beats per min), a regular rhythm, and most often, a narrow QRS complex. The P wave is usually hidden within the QRS complex. PSVT is caused by reentry, and the tachycardias are classified, electrophysiologically, according to the anatomic location of the reentry circuit. Atrioventricular nodal reentry is the most common form of PSVT. In A-V nodal reentry, there are two conducting pathways (alpha and beta) that have different conduction times and refractory periods; both pathways are confined to the A-V nodal and perinodal atrial tissue. The other common form of PSVT, termed atrioventricular reciprocating tachycardia, depends on an anatomically distinct, or "accessory," pathway that may conduct impulses between the atria and the ventricles, while bypassing the AV node. The two forms of PSVT may be distinguished in many cases by examining the 12-lead electrocardiogram. In the majority of cases of A-V nodal reentry, the atria and ventricles are depolarized simultaneously, and the P waves are hidden in the QRS complex. If the reentry circuit includes an accessory pathway, the P wave always follows the QRS, and usually the R-P interval exceeds 70 msec. Several principles should guide the management of PSVT: (a) Unstable patients require emergent electrical cardioversion; (b) A 12-lead ECG should be obtained immediately to confirm that the tachycardia has a narrow complex (ventricular tachycardia may masquerade as PSVT if only a single lead is examined); (c) Vagal maneuvers may be attempted (the Valsalva maneuver is safer and more efficacious, especially in the elderly); and (4) In most patients, adenosine is the first-line agent to treat PSVT. Contraindications to adenosine and drug interactions are noted in this article. In addition, the use of adenosine in wide complex tachycardias and the indications for admission and referral for electrophysiologic evaluation are discussed.