Clinically relevant issues of classification of intraventricular conduction disorders (IVCD), such as stability, localization and stage, are discussed in the lecture. IVCD is considered to be an important diagnostic criterion, but may limit ECG diagnosis of ventricular hypertrophy and myocardial infarction. Influence of IVCD upon clinical course and prognosis of cardiac diseases is closely related to structural and functional changes of myocardium. In many clinical situations two- and three-bundle blockades, as well as IVCD combined with syncopal episodes may predict further appearance of complete atrioventricular blockade and are considered as indications for pacing. Morphology and duration of QRS complex are key criteria for resynchronization therapy in patients with heart failure.