Pulmonary hypertension is a prevalent disease of multiple etiologies. Over the years, different attempts at nomenclature and classification, largely based on morphologic findings at autopsy, have met with limited success. Recent advances in medical and surgical treatment, and the promise of effective approaches to prevention, have prompted the development of a clinical classification designed to take advantage of the new promising interventions. This article traces the growth of understanding of the pulmonary hypertensive diseases that has enabled the development of a clinical classification oriented towards prevention and treatment.