Pulmonary hypertension can be caused by an increase in pulmonary blood flow, an increase in left-heart impedance or an increase in pulmonary vascular resistance. Although there are many specific causes that have been identified, clinical presentation and natural history are common to all etiologies. Therapeutic options remain limited to correcting underlying causes whenever possible, as well as supplemental oxygen and, in the setting of severely reduced cardiac output, anticoagulation. Vasodilatory therapy with calcium channel blockers, intravenous epoprostenol or inhaled nitric oxide has also met with some success in patients with primary pulmonary hypertension. Lung or heart-lung transplantation remains the only viable therapeutic option for patients with severe pulmonary hypertension. There have been significant advances in our understanding of the pathobiology and treatment of this uncommon but serious class of disorders. Yet, much remains to be learned in our efforts to improve the lot of these unfortunate patients.