Increasingly, the interventional cardiologist is seeking to close interatrial communications by inserting devices by means of catheterisation. So as to optimise these procedures, it is advantageous to have a firm grasp of the anatomy of the normal atrial septal structures, this then providing the basis to understand the morphology of the holes which can exist between the chambers, not all of which are true septal defects. A true septal structure can be removed without exiting from the cavities of the heart. It is the flap valve of the oval fossa, along with the anterior rim of the fossa, which fulfill this criterion. The remainder of the extensive rim of the normal fossa is no more than an infolding between the walls of the right and left atria and their venous tributaries, and has different dimensions at various points around the circumference. The so-called muscular atrioventricular "septum" is a sandwich incorporating a layer of epicardial fibro-adipose tissue. True defects of the atrial septum, therefore, exist because of deficiency, perforation, or absence of the flap valve. Most of these defects will prove suitable for interventional closure, but potential caveats include multiple defects, aneurysm of the flap valve, or adjacency of the fossa to the venous orifices. The other interatrial communications, namely the sinus venosus, coronary sinus, and "ostium primum" defects are outside the confines of the oval fossa. Recognition of this feature is the key to their diagnosis, and their differentiation from true atrial septal defects. Of these defects, only the coronary sinus defect is likely to be suitable for device closure, and then only in the very rare circumstances when it is seen in isolation.