Pulmonary hypertension (PH), defined as a mean pulmonary arterial (PA) pressure of >25 mmHg at rest or >30 mmHg during exercise, is characterized by a progressive and sustained increase in pulmonary vascular resistance that eventually leads to right ventricular failure. Clinically, PH may result from a variety of underlying diseases (Table 1 and Refs. 50, 113, 124). Pulmonary arterial hypertension (PAH) may be familial (FPAH) or sporadic (idiopathic, IPAH), formerly known as primary pulmonary hypertension, i.e., for which there is no demonstrable cause. More often, PAH is due to a variety of identifiable diseases including scleroderma and other collagen disorders, liver disease, human immunodeficiency virus, and the intake of appetite-suppressant drugs such as phentermine and fenfluramine (72). Other, more common, causes of PAH include left ventricular failure (perhaps the most common cause), valvular lesions, chronic pulmonary diseases, sleep-disordered breathing, and prolonged residence at high altitude. This classification, now widely accepted, was first proposed at a meeting in Evian, France, in 1998, and modified in Venice, Italy, in 2003 (124).