Atrial septal defect (ASD) is an open communication between the atria through a defect in the interatrial septum that allows left-to-right shunting of the blood. After a bicuspid aortic valve, atrial septal defect is the most common congenital heart disease in adulthood. Secundum ASD accounts for the majority of cases. Patients with an isolated defect often remain asymptomatic during childhood and adolescence. Most patients will become symptomatic by the age of 40 years. Exercise intolerance and dyspnea as well as supraventricular arrhythmias are the most common manifestations. Longstanding right-sided volume overload can lead to the development of severe pulmonary hypertension, bidirectional shunting or rarely to right-to-left shunting (Eisenmenger syndrome). The characteristic auscultatory finding is a wide fixed splitting of the second heart sound (S2) and systolic ejection murmur across the pulmonic valve. Transthoracic (TTE) and transoesophageal (TEE) echocardiography are the imaging modalities of choice for the diagnosis of an ASD, assessing its hemodynamic and anatomy variations. ASD should be closed in symptomatic and asymptomatic patients with signs of right heart volume overload, regardless of age but must be avoided in patients with severe pulmonary hypertension or Eisenmenger physiology. Surgery is required for closure of primum ASDs, sinus venos defects, and coronary sinus defects. Transcatheter device closure is the treatment of choice in patients with secundum ASDs with appropriate anatomic characteristics.