Cardiopulmonary exercise testing (CPET) is an important diagnostic instrument for early detection, differential diagnosis and follow-up evaluation in pulmonary hypertension (PH). A pulmonary vasculopathy as the underlying cause for pulmonary arterial hypertension (PAH) can be detected in early stages by measuring gas exchange during exercise. There are characteristic patterns which are typically seen in patients with PAH. Regarding follow-up assessments, CPET can quantify cardiopulmonary exercise capacity in a more differentiated way than global exercise tests like the 6-minute walking distance. Different pathophysiological mechanisms accounting for pulmonary hypertension can be separately evaluated by CPET. Although tremendous progress has been made regarding sensor technology and data processing, CPET is still a method that is technically challenging. In order to obtain reliable results, strict quality control is of crucial importance. Additionally, standardization of result display, independent of equipment manufacturers or institutions, is desirable, in order to ensure a uniform interpretation of results.