The efficacy of implantable cardioverter defibrillator (ICD) therapy and medical therapy in the treatment of patients with ventricular fibrillation and sustained ventricular tachycardia had been compared in large randomized studies. In this section, we aimed to summarize what we have learned from large studies on the effects of amiodarone and ICD therapy in prevention of cardiac arrest n patients with malignant ventricular arrhythmias. The analyzed studies are; The Multicenter Unsustained Tachycardia Trial (MUSTT), Antiarrhythmics vs. Implantable Defibrillator (AVID), Cardiac Arrest Stduy Hamburg (CASH) and the Canadian Implantable Defibrillator Stduy (CIDS).