Pulmonary arterial hypertension (PAH) was first described over 100 years ago, but a thorough understanding of its patho-genesis and a successful approach to curing the disease remain elusive. Increased research activity has greatly enhanced our understanding of the disease and led to new therapies that retard disease progression and improve patient function and survival. The disease is rare, but effects people of all ages and is associated with several seemingly unconnected diseases. It is not clear whether all forms of PAH represent the same disease or the same pathologic endpoint of different pulmonary vascular disease processes. Recent attempts to classify different forms of pulmonary hyper-tension by the diseases with which they are associated has helped researchers organize data collected from patient registries and clinical trials. This overview discusses the history, epidemiology, and genetic basis of PAH, new recommendations for disease classification, and prognostic variables shown to adversely affect survival.