Pulmonary arterial hypertension (PAH) is a syndrome resulting from restricted flow of blood through the pulmonary arterial circulation due to increased pulmonary vascular resistance and is complicated ultimately by right heart failure. A number of pathological pathways are involved in the development of PAH, including those at the genetic and molecular levels, and arterial smooth muscle, endothelial cells and adventitia. The imbalance in the vasoconstrictor/vasodilator milieu represents the basis for current medical therapies, although PAH also involves an imbalance of proliferation and apoptosis. The present classification of pulmonary hypertension (PH) was established in 1998 at the World Congress of PH in Evian, and was later revised in Dana Point in 2008 (Table 7.1).