During the last 15 years in arrhythmology brisk development of the use of endocardial electrocardiograms (IECG) occurred. At first the objective was to describe the characteristics of the conduction system of the heart (recording of Hisogram), next assessment of pathological features of the conduction system (such as bradyarrhythmias), electric activation of the myocardium in existing arrhythmias with the outcome of the diagnosis and accurate localization of additional pathways in endocardial mapping. The next logical step reacting to knowledge of intracardial mapping was the development of new therapeutic methods--DC and subsequently radiofrequency ablation--non-pharmacological therapeutic antiarrhythmic approaches which are by now commonly used in electrophysiological catheterization laboratories of cardiological departments. Some contemporary pacemakers can also record intracardial atrial and ventricular IECG and make it possible to use additional antiarrhythmic stimulation. A dominant position is held by the IECG in implantable cardioverters-defibrillators where on the high standard recording of IECG a considerable proportion of detection and decision-making algorithms of this equipment is based. Recording IECG is an integral part of clinical arrhythmological practice from the diagnostic as well as therapeutic aspect.