Within the framework of this work, the principle of operation of the pacemaker, one of the most important and successful medical devices, is described. In addition to a detailed description of the construction of the pacemaker as well as the function and development of its components over the years, a brief description of the physiology of a heartbeat is given. In addition, the description of the action potential is given. Moreover, the explanation of the functioning of heart activity imaging device, electrocardiograph (ECG), is given, including the description of components of a typical ECG signal. Also, the heart disorders necessitating the need for pacemaker installation are shortly listed. In addition to examining the components of the pacemaker, the reasons for the choice of materials for the manufacture of the pacemaker housing, the choice of power sources, electrodes as well as leads are explained. Furthermore, the operation of the asynchronous pacemaker, programmable pacemaker as well as the pacemaker with the response to physical activity is also described. Finally, the differences between the pacemaker and the implantable cardioverter-defibrillator (ICD) are given.