Congenital accessory pathways are the basis of AV reentrant tachycardia (AVRT). This usually presents in the form of orthodromic AVRT with anterograde conduction via the AV node and retrograde atrial activation via the congenital pathway, as a regular narrow complex tachycardia with a long VA interval. This chapter discusses the pathophysiology and presentation of AVRT, as well as the methodology of electrophysiological differential diagnosis in tachycardias with a long VA interval, as well as ablation techniques with success rates and risks of ablation. In doing so, the risk of malignant arrhythmias in patients with anterograde pathway conduction and rare forms of congenital accessory pathways with decremental conduction are addressed.