The spectrum of cardiac arrhythmias that may be encountered in paced patients is vast; as multiprogrammable and dual-chamber pacemakers have become more sophisticated, the problems of electrocardiographic interpretation have multiplied. The arrhythmias may be broadly classified as follows: those due to abnormal functioning of the device, those due to normal functioning of the device, and those due to the interaction of the pacemaker with a spontaneous arrhythmia occurring in the patient.