During normal sinus rhythm, the cardiac impulse originates in the sinus node at a rate appropriate to the age and activity of the animal and spreads in an orderly fashion throughout the atria, the atrioventricular (AV) node, the His-Purkinje system, and then throughout the ventricles. An arrhythmia is an abnormality in the rate, regularity, or site of origin of the cardiac impulse or a disturbance in conduction of the impulse so that the normal sequence of activation of atria and ventricles is altered. Cardiac arrhythmias and conduction disturbances occur in every region of the heart and are caused by numerous factors. In particular, some are aligned with certain disease states. In the final analysis, however, all arrhythmias and conduction disturbances--regardless of their pathoelectrophysiologic cause--result from critical alterations, either acute or chronic, in the electrical activity of the cardiac myocyte. This review will provide basic information on how normal cardiac electrophysiology can be changed by disease and how these changes can lead to conduction disturbances and cardiac arrhythmias.