Recent studies on the development of the heart6 have increased markedly our appreciation of the disposition of the specialized conduction tissues, showing how some rests of tissues in unexpected locations are remnants of a well organized system found throughout avian and mammalian species. These studies also help us to account more logically for the distribution of the conduction tissues in congenitally malformed hearts. On the basis of our overall studies, therefore, combined with these recent advances, we are able to understand the normal and abnormal disposition of the conduction tissues. These understandings then permit us to offer explanations for those arrhythmias which have a “natural” basis, such as the grossly abnormal patterns found in hearts with isomerism of the atrial appendages or univentricular atrioventricular connection, and the accessory pathways found in the setting of ventricular preexcitation. We are also able to understand the changes affecting these tissues as part of acquired disease, such as the fibrosis of the long atrioventricular bundles found in atrioventricular septal defect and corrected transposition and the immunologic damage underscoring congenitally complete heart block in infants of mothers with collagen disease. We are only beginning to explore the possibility that abnormalities of the myocardiutn itself, or the fibrous tissue matrix, can also underscore the existence of abnormal rhythms. With all this knowledge, it is to be hoped that, in the future, we will see far fewer of those arrhythmias which have an “unnatural” basis, such as those related to direct surgical damage to the conduction tissues and their arterial supply, or those produced by abnormal loading conditions which are themselves the consequence of congenital malformations. Although we have learned, and applied, a great deal, there is still much to be done before we unravel all the secrets of the causes of arrhythmias in childhood.