Available evidence suggests that the ion channels that generate the normal action potential are also the basis for the arrhythmias that occur in disease states. Therefore, a thorough understanding of the function of the ion channels that generate the action potential is an important foundation for understanding the bases of arrhythmias and their treatment. This need is made all the more pressing by the discoveries in molecular genetics and membrane biophysics that have elucidated the fundamental mechanisms of a broad range of cardiac arrhythmias.