In the United States, approximately 30,000 children are born with congenital heart disease every year. Between 1000 and 2000 of these children have some form of functional single ventricle anatomy, with variants of hypoplastic left heart syndrome (HLHS) comprising about half. Patients with functional single ventricle anatomy are born with a malformation that renders one of their ventricles unusable and surgically unrecoverable. In HLHS, for example, either the mitral or aortic valve can be absent or significantly stenotic, which is generally associated with marked hypoplasia of the left ventricle and the ascending aorta. Other common variants of single ventricle anatomy are tricuspid or pulmonary atresia, often associated with some degree of right ventricular hypoplasia, unbalanced atrioventricular defects, in which either ventricle can be unusable due to malposition of the interventricular septum, and the heterotaxy syndromes. All of these congenital cardiac anomalies, as well as several other less common variants, can be treated with a multi-stage surgical palliation that usually requires three surgical procedures during the first three years of life. At the completion of this series of palliative procedures the children are left with their single functional ventricle driving blood flow to the systemic circulation while their pulmonary circulation is derived from passive drainage of the systemic venous return into and through the pulmonary vasculature. This arrangement, the so-called "Fontan physiology", can provide an excellent quality of life as these children grow and enter young adulthood; however, it is by no means a normal hemodynamic arrangement and a significant percentage of children with single ventricle anatomy will fail either during the early palliative steps or at some point after they achieve final Fontan palliation.