The implantable cardioverter/defibrillator (ICD), which uses antitachycardia pacing or shock, is the only adequate therapeutic option for life threatening ventricular tachyarrhythmias. The results of prospective, randomised ICD trials show that the indications for primary and secondary prevention of sudden cardiac death have changed in a relevant manner. After the identification of a patient at risk, the implantation of an ICD reduces cardiac and total mortality significantly. A clear diagnostic procedure and the parameters needed for the identification of this risk are essential. We report on the indications for the implantation of an ICD following current guidelines in order to enable the physician to provide his patients with an adequate ICD therapy in everyday practice.