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The term “preexcitation” indicates the premature activation of a portion of myocardium by an impulse traveling an anomalous pathway, able to conduct electrical impulses faster than the normal conduction pathways. Usually such anomalous, accessory pathways connect the atria to the ventricles and allow an impulse originated in one chamber (atrium or ventricle) to reach the other without the physiological delay elicited by the atrioventricular node. The accessory pathways are constituted in most cases by fast, sodium-dependent fibers with rapid conduction capacity. Differently, the atrioventricular node is constituted by calcium-dependent fibers with a slow conduction capacity. The term “preexcitation” is not a synonymous of “ventricular preexcitation”: in fact, in the presence of an accessory pathway, the atria can be pre-excited by an impulse originating from the ventricles without any effect on the ventricular activation. Cardiac preexcitation Localization of the accessory pathway at surface ECG Arrhythmias in preexcitation The endocavitary electrophysiological study How to identify patients at risk Therapy of arrhythmic episodes Accessory pathway ablation Permanent junctional reciprocating tachycardia (PJRT) or Coumel tachycardia Mahaim fiber tachycardia Clinical Cases