This issue of Cardiac Electrophysiology Clinics, Guest Edited by Drs. Giuseppe Bagliani, Roberto De Ponti, and Fabio Leonelli is dedicated to The Complex World of Cardiac Pre-Excitation: towards Precision Electrocardiology. This is one of four issues selected each year by the series Consulting Editors, Ranjan K. Thakur and Andrea Natale. Topics include, but are not limited to, Anatomy of the AV junction, Pre-excitation, Algorithms to identify accessory pathways location on 12-lead ECG, Pathophysiology of accessory pathways-mediated arrhythmias, Arrhythmias with by-stander accessory pathways, Arrhythmias involving unusual accessory pathways, Sudden cardiac death in patients with ventricular pre-excitation, Clinical approach to symptomatic and asymptomatic patients with ventricular pre-excitation, Methodology of accessory pathway ablation, Ablation of accessory pathways with unusual anatomic location, Ablation of accessory pathways with uncommon electrophysiologic properties, and Ablation in pediatric patients and in association with congenital heart disease.