The term “univentricular (single ventricle) heart” refers to many complex congenital heart defects (CHD). The main characteristic of these malformations is the presence of a single anatomical or functional chamber. Normally, the cardiovascular system consists of two circles which are connected in series, i.e. the pulmonary and the systemic circles are driven by a “double pump” – the right and left ventricle. In the univentricular heart, the single ventricle maintains both systemic and pulmonary blood fl ow. The two circulation circles are connected in parallel rather than in series, as is the case in the normal circulation which is driven by two ventricles. The classic Fontan operation was introduced 50 years ago. The Fontan type operations are staged procedures aimed at complete separation of the systemic and venous circulation without a subpulmonary chamber in univentricular CHDs. Originally designed for patients with tricuspid atresia, this concept is currently used i n many complex CHDs in which two ventricular correction is not possible. Although it divides the blood circles and eliminates cyanosis, this physiological correction is associated with signifi cant changes in hemodynamics. The cardiologists and pediatricians should be aware of these peculiarities of the Fontan circulation.