A left-to-right shunt lesions exists when blood from the left atrium, left ventricle, or aorta transits to the right atrium or its tributaries, the right ventricle, or the pulmonary artery. This article discusses: the incidence, types, embryology, clinical presentations, physical examinations, electrocardiographic features, chest radiographs, echocardiographic and cardiac catheterization issues, treatment, natural history of atrial septal defects, and outcomes of treatment of atrial septal defects, ventricular septal defects, patent ductus arteriosus, and endocardial cushion defects.