**1. Introduction**

The detection of cardiac arrhythmia and the differential algorithm used for the recognition of ventricular tachycardia (VT) or ventricular fibrillation (VF) necessitating internal cardiac de‐ fibrillator (ICD) therapy is one of the most critical issues in ICD patients. Main goal is to devel‐ op algorithms with the highest possible sensitivity to avoid untreated arrhythmias and highest possible specificity to avoid inappropriate ICD therapies. Currently, several device discrimi‐ nation algorithms and their combinations are available. The following chapter tries to explain the functionality, means, possibilities and restrictions of these ICD discrimination algorithms.
