Severe pulmonary hypertension has a poor prognosis and is complicated to treat. It is caused by states of hypoxia, thromboembolism, chronic inflammation, pulmonary venous congestion and/or hypercirculation. In the course of some days up to many years, remodelling of the pulmonary vasculature may occur. Morphologic characteristics of the remodelling process have extensively been described, while the underlying cellular and molecular mechanisms remain largely unknown. Increasing knowledge of the features of pulmonary vasoconstriction and vascular remodelling, will, however, improve the options for future therapy.