Hypoplastic Left Heart Syndrome (HLHS) was first described fifty years ago. It is characterised by underdevelopment of the left ventricle with associated aortic and mitral valve hypoplasia or atresia, and varying degrees of hypoplasia of the aortic arch (Figure 1). In utero a physiological right to left shunt of oxygenated blood through the ductus arteriosus bypasses the obstruction and allows normal fetal growth. Closure of the ductus arteriosus in the post-natal period interrupts systemic blood flow, resulting in rapid deterioration and death. Untreated it is a universally fatal condition, leading to neonatal death within the first few days or weeks of life. Although HLHS affects only one baby in 10,000 and accounts for less than 10% of all congenital heart disease, HLHS is responsible for 25% of all deaths due to congenital cardiac disease occurring within the first week of life.