The science of the electrical activity of the heart. The heartbeat results from the development and organized control of cardiac excitability, including ionic current flow across the cardiac membrane, within and between cells, and throughout the body, which in turn allows the orderly contraction of heart muscle and the efficient pumping of blood. Alterations in these determinants of normal cardiac excitability may lead to abnormalities in the rhythm of the heart (arrhythmias or dysrhythmias) and in the propagation of electrical impulses throughout the heart (conduction defects). Cellular electrophysiologic events lead to the establishment of extracellular potentials on the surface of the body. Electrocardiographic, vectorcardiographic, and other recording systems are used to determine the orientation and magnitude of that extracellular potential. The normal heart produces characteristic sequences of extracellular potentials that may be altered by disease.