Univentricular heart defects are among the most complex congenital malformations. The treatment of these patients usually includes 3 operations and accompanying diagnostics, some of which are invasive, within the first 3 to 4 years of life until the fontan circulation is established. Unfortunately, the group of patients with a univentricular cardiac malformation is also the group with the highest mortality rate until the Fontan circulation is established. The expected mortality varies depending on the complexity of the underlying intracardiac and extracardiac malformations and possible concomitant morbidity; According to data from the National Quality Assurance in Germany, it is currently up to 30% in patients with hypoplastic left heart syndrome. However, the treatment methods are well standardized and there has been a clinical protocol in Erlangen since 2008 that includes diagnostic examinations using cardiac catheterization (HKU) and magnetic resonance imaging (MRT), as well as non-invasive, instrument-based diagnostics and targeted blood tests in a consistent sequence before and after the operations regulates. The Erlangen protocol ends with the cardiac MRT six months after the Fontan operation (syn: total cavopulmonary connection \[TCPC\]). If MRT imaging is not possible (e.g. if a cardiac pacemaker is present) or if there is a clinical indication (e.g. an obstruction noticeable on MRT or the presence of an overflow with right-left shunt), HKU is performed in the first year after TCPC. These examinations are used to depict hemodynamics, cardiac function, vascular development, and lymphatic drainage disorders in order to evaluate the preoperative risk, adapt treatment strategies to the individual, and identify complications at an early stage. This concept is based on clinical experience and current scientific findings and has proven itself in everyday clinical practice since its introduction for Erlangen-Fontan patients and has proven to be very helpful for individualized treatment. After a successful Fontan operation, the mortality risk in childhood is only low. In the long term, however, some of the patients unfortunately suffer from Fontan-typical long-term complications with protein-loss enteropathy having to be mentioned in particular, which is described in the literature with an incidence of 3-14% and today still a 5-year incidence-has a mortality risk of 6-12%.