This article examines the role of electrocardiography in patient monitoring during an operative procedure. In addition to providing a wealth of physiological information, including information on the electrical activity of the heart, the ECG assists in monitoring and detecting a variety of changes, such as cardiac arrhythmias, electrolyte changes, and ischemia. Information presented on an ECG should be analyzed systematically with an understanding of the constituent elements of an ECG, the rate, the rhythm, the morphology, the axis, the presence of conduction abnormalities, electrolyte changes, and ischemic changes. To assess accurately the information presented, hemodynamic information and cardiac-risk analysis should be integrated to have a complete picture.