Often assimilated to simple inter-atrial communication, the ostium primum, or partial atrio-ventricular septal defect, is an entity that is characterized by a different embryological mechanism and requires some specific surgical expertise. Basically, knowledge of the morphology of the common atrioventricular valve with 5 components, the topography of the A-V node and His bundle, and the ventricular consequences of the absence of atrio-ventricular septal structures must be taken into account.