Defibrillation and cardioversion are techniques in which a short electric impulse is administered to the heart in order to restore its normal rhythm. During cardioversion electric impulse is synchronized to the QRS on electrocardiogram. During defibrillation electric current passes through the heart in any phase of electric heart cycle. This mode of treatment is rather new - Lown et al. started to use it in a clinical practice in 1962. During defibrillation or cardioversion electric current goes from negative to positive electrode of defibrillator and passes the heart on its way. This induces transmembrane potential in myocardium cells and results in synchronic depolarization of all myocardium. The pathophysiology of defibrillation is explained by critical mass hypothesis as well as the upper limit of vulnerability hypothesis. The success of defibrillation depends on many factors, such as the location and size of electrodes, the type of defibrillator, the morphology of electric impulse, transthoracic impedance, the type and duration of arrhythmia. This procedure can be performed only on unconscious patient. The possible complications of the procedure can be disturbances in heart rhythm and conduction, the changes in arterial blood pressure, the damage to the myocardium, embolia, pulmonary edema and others. This article describes the mechanism of action of defibrillation and cardioversion, indications for this procedure, the technique and methods of defibrillation and cardioversion, the factors, responsible for the efficacy of the procedure and possible complications of defibrillation.