A clear understanding of the components of the pacemaker timing cycles is crucial to understanding and interpreting paced electrocardiograms. The information given in this article provides basic rules for timing cycles for pacing modes currently in use and pacing modes less frequently used but of importance in understanding current timing cycles. Although understanding basic timing cycles allows interpretation of most paced electrocardiograms, manufacturer-dependent variations of the timing cycle operation make it necessary to be intimately familiar with the design of each pacemaker to be interpreted.