Paroxysmal supraventricular tachycardia (PSVT) is defined as a regular rapid heart beat, which initiates and terminates suddenly. PSVT may have a variety of electrophysiologic mechanisms, including atrial tachycardia, atrioventricular (AV) nodal reentry, and tachycardia involving an accessory AV connection. Atrial tachycardia may be reentrant or may be caused by abnormal automaticity or triggered activity. AV nodal reentry, in which the reentrant circuit is confined to the AV node or the region around the AV node, is the most common type of PSVT in adults. Orthodromic supraventricular tachycardia is the most frequently found tachycardia in patients with accessory AV connections. During orthodromic supraventricular tachycardia, antegrade conduction occurs via the AV node, and retrograde conduction occurs via an accessory AV connection. Other types of PSVT, including junctional tachycardia, are less common. A definitive diagnosis of the PSVT mechanism usually requires electrophysiologic testing, but clinical and electrocardiographic clues may be present. Understanding the mechanism of PSVT can alter therapy because the response to antiarrhythmic drugs may be somewhat different depending on the PSVT type. In addition, the risks and efficacy of catheter ablation for curing PSVT may differ depending on the PSVT type. A better understanding of PSVT mechanisms, which has developed over the past 20 years, has led to dramatic improvements in therapy.