A major problem in patients with cardioverter-defibrillators is to determine reliably the mechanism for spontaneous implantable cardioverter-defibrillator (ICD) discharges. Electrogram storage in ICD devices is comparable to that in permanent Holter monitors. Stored bipolar electrograms obtained from the sensing or shocking lead system contain a wide variety of different information. Intracardiac electrograms (EGMs) recorded from pace/sense electrodes (or “near-field” EGMs) show bipolar signals that have a distinct absence of any atrial activity during ventricular tachycardia or sinus rhythm. In contrast, the EGMs recording from the shocking electrodes, integrating a much larger area of myocardium, provide a more global visualization of electrical activity, which includes both atrial and ventricular deflections. Improved diagnostic capabilities available in the new generation ICD devices, in particular the stored intracardiac EGMs, facilitate sensing error diagnosis, permit a better evaluation of device function and ICD detection algorithms, and are helpful for reprogramming in order to overcome or prevent errors. In addition, EGMs give us a unique opportunity to gather information about the arrhythmic mechanism of the sudden cardiac death syndrome. Information such as the day and time of the episode, the preceding heart rate, the influence of the coupling interval of preceding premature beats, and their morphology can be gained from the analysis of stored EGM recordings. Although the availability of stored intracardiac EGMs are of enormous value in troubleshooting of ICD problems, they are occasionally not conclusive and must be complemented by additional techniques in order to complete the diagnosis. The information obtained by the analysis of stored intracardiac EGMs together with a database of EGMs can be of great importance for further improvements in future devices and may provide insights as to which patients are likely to benefit most from ICD therapy.