Accessory atrioventricular anomalous conduction is responsible for pre-excitation in electrocardiogram (ECG) — a typical feature of Wolff–Parkinson–White syndrome (WPW). The PhD thesis aimed to analyze the role of the electrocardiogram as a diagnostic tool in WPW. The study was designed as a retrospective analysis of accessory pathway ablation procedures. The study confirmed that SPRRI and arrhythmia inducibility can be helpful to identify patients with a high risk of malignant arrhythmias; however, its predictive value does not seem satisfactory. Lower threshold for prophylactic ablation should be used to prevent WPW-related adverse events. It was shown that the greater pre-excitation in ECG improves the accuracy of the traditional AP localizing algorithms. Pambrun algorithm designed to use with maximally preexcited ECG had the best overall accuracy. Preferably maximally pre-excited ECGs should be used in clinical practice to facilitate the ablation procedure. In addition, the study suggests that there are sufficient electrocardiographic differences between VT and preexcited SVT to allow electrocardiographic differentiation. VT score, Steurer algorithm, and some single criteria do not overdiagnose VT in patients with preexcitation.