Pulmonary arterial hypertension (PAH) is a term used to classify a variety of conditions that have in common an injury to the pulmonary vasculature that produces elevations in pulmonary arterial pressure. There have been considerable advances in our understanding of the pathogenesis and treatment of PAH over the past decade. The article reviews the classification of diseases associated with PAH, the current understanding of its pathogenesis, and the contemporary approach to therapy.