The natural history of atrial septal defect is well established with development of pulmonary hypertension and its complications in larger defects with substantial left-to-right shunting. While such a problem does not necessarily occur in patients with small defects and patent foramen ovale, they might present with other problems, like strokes, hypoxia and severe decompression sickness from right-to-left shunting. Over the last decades, our knowledge about the risk of those anomalies and optimum treatment has significantly improved, thanks to the development of imaging techniques that allow accurate assessment of the defects in context of overall cardiac structure and function. This chapter deals with the different types of secundum ASD and PFO, and the role of echocardiography in their optimum closure strategy.