Pacing system malfunction, although seemingly difficult to assess, can be categorized in relation to the dysfunction of the leads or the generator and apparent dysfunction related to the idiosyncratic characteristics of the pacemaker's timing algorithms. In contrast to the relative frequency of lead failure as a result of implantation error or deterioration of the lead materials, primary malfunction of the pulse generator is rare. Patient-specific problems or inappropriate program settings are relatively common. Consequently, the keys to understanding unexpected pacemaker behavior are meticulous evaluation of the integrity of the leads, assessment of capture and sensing thresholds, and an understanding of the timing cycles of the specific pacemaker, which is facilitated by access to event marker telemetry. Clues to the problem and its cause are founding the patient's history, physical examination, and the various diagnostic tests integral to the pacemaker that are retrieved through bidirectional telemetry. With respect to the hardware, the answer is usually lead dysfunction or a behavioral eccentricity detailed in the pacemaker's technical manual. One must always keep the patient's physiology and pathophysiology in mind, because they also affect the function of the pacing system. Furthermore, even if all components of the system are normal, the pacemaker may be programmed to a set of parameters that are no longer optimal for the patient. When the clinician presented with a suspected pacing system malfunction, it is essential to proceed in a meticulous and orderly manner, carefully assessing, each component of the system, including the pulse generator, the programmed settings, any unique algorithms, the leads, and the patient. If a complete assessment of capture and sensing thresholds, lead impedance, sensor response, and the behavior of any unique algorithms is performed on a periodic basis as part of the routine surveillance of the patient's pacing system, baseline data will be available for comparison with the results of evaluation when and if a problem is suspected.