Basic cardiac electrophysiology is foundational to understanding normal cardiac function in terms of rate and rhythm and initiation of cardiac muscle contraction. The primary clinical tool for assessing cardiac electrical events is the electrocardiogram (ECG), which provides global and regional information on rate, rhythm, and electrical conduction as well as changes in electrical activity associated with cardiac disease, particularly ischemic heart disease. This teaching review is written at a level appropriate for first-and second-year medical students. Specific concepts discussed include ion equilibrium potentials, electrochemical forces driving ion movements across membranes, the role of ion channels in determining membrane resting potentials and action potentials, and the conduction of action potentials within the heart. The electrophysiological basis for the ECG is then described, followed by discussion on how ischemia alters cellular electrophysiology and ECG recordings, with particular emphasis on changes in T waves and ST segments of the ECG.