This study is inspired by the gap in knowledge regarding the timing of cardiac surgery and interventions in adult patients with congenital heart disease. There are many parameters used assessing right ventricular function; however, most of them have pitfalls. Understanding the pathomechanisms by which the heart adapts to congenital defects is probably key to find the answer when it is time to intervene and start discussing treatment options. Heart defects are the most frequently occurring congenital disorders. Less than 50% of individuals with moderate to severe congenital heart defects, e.g. transposition of the great arteries (TGA) or tetralogy of Fallot (TOF), survive to adulthood without intervention. Advances in cardiac surgery and better identification of individuals at risk for sudden cardiac death have increased survival rates. Currently, more than 96% of patients with congenital heart disease survive to at least 16 years of age; most undergo corrective surgery but are not cured, and only a few have normal physiology and anatomy. In many cases, the heart must develop mechanisms of adaptation to the changed conditions after surgery. Consequently, correction of the defect creates residual disease with a risk of future complications. To prevent clinical deterioration and to identify the development of complications, patients need lifelong, regular follow up. The choice of followup modalities depends on the cardiac malformation. The right ventricle (RV) plays an important role, as it is often part of the defect or is influenced by the surgery. In the past, research was focused on assessment of left ventricular function (LV), and the RV was “the forgotten ventricle.” Observations and studies in the last few decades brought increased interest into the RV and revealed the importance of the RV in the prognosis of various cardiac diseases. An understanding of RV morphology, pathophysiology and adaptive mechanisms is crucial for further studies of prognosis as well as for research linked to the use of particular diagnostic modalities. When the RV is exposed to increased pressure load, e.g. in atrially corrected transposition of the great arteries (TGA), adaptation affects the cavity volume as well as the wall thickness. When the RV is volume overloaded, adaptation involves enhancement of the RV cavity volume while the wall thickness often remains unchanged under long time. RV ejection fraction (RVEF) gives some information about changes in RV function, but information on myocardial contractility and contractile reserve is also needed. New functional parameters such as strain—also known as myocardial deformation—provide some information about intrinsic myocardial function. In Paper I, we studied functional parameters such as ejection fraction and strain (radial and longitudinal strain for both ventricles) in patients with Tetralogy of Fallot (TOF) and TGA. Longitudinal RV strain was depressed in both patient groups in comparison with that in healthy individuals, and there were additional differences between the two patient groups. In Paper II, we validated three-dimensional echocardiography (3DEcho) against the cardiac magnetic resonance (CMR) gold standard. The study population was limited to patients with TOF. In general, 3DEcho underestimated RV volumes but was able to identify patients with RV dilatation on CMR with high sensitivity. RV longitudinal free wall strain measured by CMR with a cut-off set at -14% identified patients with depressed exercise capacity and low peak oxygen uptake. In Paper III, we studied a new CMR method to quantify and visualise turbulent flow in the heart and vessels. Turbulent flow can be harmful to tissue, blood cells, and endothelium and can contribute to tissue remodeling. In patients with TOF, turbulent flow can be seen as variance in 2DEcho color Doppler. In CMR, increased turbulent kinetic energy (TKE) could be seen with four-dimensional flow. The RV TKE was increased in patients with TOF with pulmonary regurgitation compared with that in healthy controls. In Paper IV, we validated “knowledge-based reconstruction” (KBR), a novel method to calculate RV volume, against CMR in patients with various types of congenital heart defects. Two-dimensional echocardiogram-based threedimensional RV reconstruction is a relatively uncomplicated method that creates a three-dimensional RV model based on a limited number of predefined points of interest (RV structures such as tricuspid annulus, RV free wall, or pulmonary valve). KBR showed good agreement with CMR (intraclass correlation coefficient = 0.84 for RV end-diastolic volume and 0.89 for ejection fraction) but tended to underestimate RV volumes, which is in line with other methods based on ultrasound. Conclusions: 3DEcho is an evolving modality that is able to identify patients with RV dilatation. It can be used clinically for the follow up of patients with congenital heart diseases, especially those with mildly to moderately dilated RVs. When an intervention seems likely, 3DEcho results should be verified by CMR. CMR-derived measurements of longitudinal and radial strain provide a new understanding of RV remodeling and ventricular interdependence in patients with TOF and TGA. Depressed longitudinal strain may indicate a risk of depressed exercise capacity and, in patients with TGA, clinical deterioration. Further studies in larger populations of patients with congenital heart defects are needed, as the altered RV morphology in such patients makes quantitative assessment especially challenging.