Supraventricular tachycardias (SVTs) are common. Reentry is the most common of the underlying mechanisms. The most frequently observed narrow QRS complex SVTs are atrioventricular nodal reentrant tachycardia and atrioventricular reentrant tachycardia using an accessory pathway. All reentrant tachycardias share certain characteristics that distinguish them from automatic or triggered tachycardias. These characteristics include unidirectional block, delayed conduction, and recovery of excitability within an intact circuit. The characteristics of the reentrant circuit and the physiology of the pathways can be used to define treatments that may be efficacious and/or may have potential risk. Differentiation of SVTs is possible with careful application of monitoring, history taking, and electrophysiologic interventions such as programmed stimulation.