Knowledge of the embryological development of the interatrial septum can provide the basis for understanding the morphogenesis of some congenital cardiac malformations. The interatrial septum consists of several embryological components including the septum primum, the anterosuperior and the posteroinferior cushions of the atrioventricular canal, the septum secundum, the left sinus valve, and the “spina vestibuli.” Atrial septation starts with the formation of the septum primum and the space between the free edge of the septum primum, and the atrioventricular cushion is the ostium primum. Before the closure of the ostium primum, multiple perforations appear in the septum primum, and their coalescence forms the ostium secundum. The septum secundum grows to the right of the septum primum. The complex of the lower rim of the septum secundum and the ostium secundum is called foramen ovale. During the fetal period because of the higher pressure in the right atrium, the blood passes from the right to the left atrium. After birth, when lung circulation begins its function, the pressure in the left atrium rises, and the foramen ovale is closed by the septum primum.