The rapid recognition, diagnosis, and management of cardiac arrhythmias are essential skills in the intensive care unit (ICU). Outcomes can hinge on the timely initiation of arrhythmia therapy. The aim of this chapter is to serve as a useful, practical resource by: (1) describing the basic physiology of the normal cardiac rhythm followed by the common mechanisms causing rhythm disturbances, (2) describing common tools used to diagnose rhythm, (3) describing specific pediatric arrhythmias, causes, and treatments, and (4) providing a practical guide to pacemakers and pacing in the ICU setting.