The most common defects in the atrial septum are ostium secundum, ostium primum and sinus venosus atrial septal defects (ASDs) and patent foramen ovale. The management of ostium primum and sinus venosus defects is by surgery because of associated abnormalities, namely, cleft in the mitral valve causing mitral regurgitation in ostium primum defects and partial anomalous pulmonary venous connection in sinus venosus defects and is addressed in Chapter 1. Patent foramen ovale (PFO) in relation to presumed paradoxical embolism, platypnea-orthodeoxia syndrome, migraine, decompression illness and others may also require closure and the considerations for closure of such PFOs are different than those of closure of ostium secundum ASDs and some of these are discussed in other chapters in this book and will not be addressed in this chapter. In this chapter only ostium secundum ASDs in adult subjects will be discussed; I will address issues related to why, when and how should atrial septal defects be closed in these subjects. The methods of transcatheter closure in adults will also be reviewed as are the approaches to occlude complex forms of ASD.