Cardiac catheterization with direct measurements of pressure and saturation in cardiac chambers and great vessels and calculation of systemic and pulmonary blood fl ow, pulmonary and systemic resistance, and pulmonary vascular reactivity are the gold standard for assessing the hemodynamics of congenital heart malformations with a left to right shunt in both children and adults. They are the basis for determining the indications for surgical or interventional treatment. Assessment of the blood fl ow according to Fick’s principle and the resistance according to Ohm‘s law are the theoretical basis of hemodynamic calculations. The steps in assessing hemodynamics in a patient with a left-to-right shunt are presented consequently: pulmonary to systemic blood fl ow ratio, calculation of pulmonary vascular resistance (PVR) in case of pulmonary hypertension (PHy), and assessment of pulmonary vascular reactivity and their clinical signifi cance as well. Special attention is paid to the assessment of hemodynamics and indications for closure in elderly patients with atrial septal defect (ASD) and left ventricular dysfunction. The limitations and sources of errors in hemodynamic calculations are presented.