Implantable cardiac defibrillators (ICD) have evolved prominently since their introduction in clinical practice in 1980. The indications for implant of the original “shock boxes” were limited to secondary prevention after the survival of at least two episodes of sudden cardiac death. The devices delivered only high-energy monophasic shocks between pericardial patches implanted by cardiac surgeons. Today’s practice guidelines, based on large clinical trials, include primary prevention as an indication for implantation of ICD, and the devices are most commonly implanted by cardiologists, using endocardial leads. Antitachycardia pacing therapy limits the need to deliver shocks, and the programmable shock polarity and waveforms have increased the efficacy of the system. ICD have become extremely complex and offer a large number of programmable parameters, whether for cardiac pacing, the memory functions and the various therapies. Because of this technologic evolution and the characteristics of devices available from a single manufacturer or among various manufacturers, the acquisition of knowledge is timeconsuming and constantly changing. It is, nevertheless, indispensable to be thoroughly familiar with the nuances of each model in order to confirm or dispute the diagnosis made by the ICD regarding each stored event, as well as, perhaps, reprogram the parameters with a view to adapt them specifically to the characteristics of individual device recipients. This original monograph covers the indications for ICD and broad principles of their function using clinical cases and an atlas of tracings meticulously selected from recipients of Saint Jude Medical defibrillators. The first tracings familiarize the readers with the various markers and with the specificities of the manufacturer with respect to diagnosis, discrimination, therapies and the memorization of the episodes. Once familiar with these characteristics, the reader can focus on clinical examples and on the specifics encountered during the follow-up of an ICD patient. The interpretation of the tracings is didactically detailed by systematically describing the multiple stages that lead to the final diagnosis. A practical comment, thereafter, offers a solution to the problem posed, and presents possible programming changes adapted to the specific characteristics of each patient.