The ablation techniques, which derive from surgical approach in the treatment of ventricular tachycardia, were originally based on the modification of conduction in the so-called site of origin of the arrhythmia. A reentrant tachycardia involves two areas of conduction, one which consists of normal tissue, and the other with slow-conduction properties isolated from the previous structure. We now think that a more appropriate place to deliver the shock should be the area of slow conduction. The purpose of this paper is to review the main electrophysiological characteristics of this tissue, which could be interesting markers to identify the site where the shock should be delivered.