Pumping of the heart is ultimately the macroscopic manifestation of coordinated contraction by the organ's individual cardiomyocytes. One of the major intellectual concerns of cardiac physiology has been to understand how cardiomyocyte contraction is initiated and how the contraction of millions of cardiomyocytes is coordinated to achieve regulated pumping of blood. It is now known that the basis of both initiation and coordination of cardiomyocyte contraction is the rhythmic and regulated electrical depolarization of the cardiomyocyte membrane. Depolarization of the cardiomyocyte membrane initiates cellular contraction and the regulated propagation of depolarization throughout the organ is responsible for the heart's coordinated contraction. The field of cardiac electrophysiology is focused on understanding the molecular basis of cardiomyocyte depolarization, how depolarization is coordinately spread throughout the organ, and how rhythmic waves of depolarization are achieved.