Implantable cardioverter defibrillators deliver shocks in response to electrical signals that satisfy programmed criteria for detection of VT or VF. The first step in diagnosis of inappropriate shocks in patients with ICDs is to determine if the shock was delivered in response to a true tachyarrhythmia by inspecting data stored in the ICD. Shocks occur in the absence of tachyarrhythmias because nonarrhythmic physiologic or nonphysiologic signals are oversensed by the ICD and detected as arrhythmias. Diagnosis and causes of oversensing are reviewed. The second step in diagnosis is to determine if the tachyarrhythmia stored in the VT/VF episode log is VT/VF or SVT by analyzing stored electrograms. Frequent or repetitive shocks constitute an electrophysiologic emergency. The approach to this problem is reviewed.