More than 30,000 patients in the United States have implanted devices that cardiovert, shock, or pace their hearts during a dangerous ventricular arrhythmia. This number grows daily as these devices are increasingly able to provide effective treatment for dangerous ventricular arrhythmias. In the past 5 years, the technology surrounding these devices has grown dramatically, going from a single "shock box" made by one vendor to a sophisticated group of devices with multiprogrammable functions. Clinicians who provide care to these patients must be able to understand the purpose of the device and validate its function. A guide to each of the implantable cardioverter defibrillator systems and to the various therapies available in each is presented.