Modern cardiac pacing systems have sophisticated software to document, evaluate and record intrinsic and paced rhythms as well as correct pacing abnormalities and rhythm disturbances by applying algorithms, which are generally company specific. To the cardiologist and technologist, these algorithms may be difficult to interpret on both the 12-lead electrocardiograph (ECG) and Holter ambulatory monitoring recordings, which are usually performed because of patient symptoms or physician concern. The tracings may appear bewildering and mimic pacemaker malfunction, thus leading to unnecessary tests or even surgery. This review will define the common programmed pacemaker modes and describe a range of ECG appearances of normal pacemaker function during the application of testing, correcting or therapy algorithms.