Sudden cardiac death is a major health problem in most industrialized countries around the world including Thailand. It is mainly caused by ventricular fibrillation (VF). Currently, defibrillation is the only effective clinical treatment of this fatal arrhythmia. Although defibrillation mechanism has been investigated extensively for many decades, its definite mechanism is still debated. It is known that understanding the basic mechanism of defibrillation is essential to develop better treatment of VF: In the present article, seven hypotheses commonly proposed as the mechanism of ventricular defibrillation are reviewed. Since research in the field of defibrillation mechanism is dynamic, the present review is to update the information to clinicians and basic investigators on the mechanism of defibrillation available to date.