Pulmonary arterial hypertension (PAH) is an uncommon disorder that may be hereditable, idiopathic or associated with conditions like drug exposure, connective tissue disease, HIV infection or congenital heart disease. Familial disease are usually due to mutations in the bone morphogenic protein receptor type 2 (BMPR2), activin-like kinase-type 1 (ALK1) and endoglin (ENG). Functional and structural changes in the pulmonary vasculature lead to increased pulmonary vascular resistance. Vascular remodeling involves endothelial dysfunction, activation of fibroblasts and smooth muscle cells and recruitment of circulating progenitor cells. Vasoconstriction has also been shown to affect the remodeling process. Genetics, cellular and molecular basis of PAH are discussed in the paper.