Orderly contractions of the atria and ventricles are regulated by the transmission of electrical impulses that pass through modified cardiac muscle cells (the cardiac conduction system) interposed within the contractile myocardium. This intrinsic conduction system is composed of specialized subpopulations of cells that spontaneously generate electrical activity (pacemaker cells) or preferentially conduct this activity throughout the heart. Following an initiating activation (or depolarization) within the myocardium, this electrical excitation spreads throughout the heart in a rapid and highly coordinated fashion. This system of cells also functionally controls the timing of the transfer of activity between the atrial and ventricular chambers. Interestingly, a common global architecture is present in mammals, with significant interspecies differences existing at the histological level (1,2).