Principles of the electrocardiographic diagnosis of ventricular preexcitation syndromes are discussed in the lecture. Contemporary classification of accessory pathways and associated preexcitation syndromes is provided. ECG signs of Wolff – Parkinson – White syndrome and criteria of its differential diagnosis with other diseases are presented. Based on clinical examples, possibilities and limitations of the existing algorithms to establish accessory pathways localization are analyzed. Most prevalent paroxysmal tachycardias appearing in preexcitation syndromes are shown. Special attention is driven to the decreased role of drug treatment and priority of catheter interventions.