In 1980, therapy with implantable cardioverter-defibrillators in survivors of cardiac arrest was introduced by Michael Mirowski. Over the last 15 years technology in these devices has dramatically improved. In this overview we give a short historical perspective and discuss, briefly aspects of generators, lead systems, sensing and detection algorithms as well as the main therapies of the ICD. Several studies have demonstrated that ICD therapy nearly eliminates sudden cardiac death and that device- and procedure-related mortality has become very low. Despite these impressive results, the overall benefit in terms of mortality has to be assessed; therefore, several randomized studies comparing ICD and competing pharmacological therapies have been started. Because of the rapid growth in popularity of these devices, guidelines for their use have been developed.