A clinically useful, treatment-based classification of pulmonary hypertension divides the disease into 5 distinct categories: (1) pulmonary hypertension associated with disorders of the respiratory system and/or hypoxemia; (2) pulmonary venous hypertension; (3) chronic thromboembolic disease; (4) pulmonary arterial hypertension; and (5) pulmonary hypertension due to disorders directly affecting the pulmonary vasculature. Pulmonary arterial hypertension includes individuals with primary pulmonary hypertension, congenital heart disease, connective tissue disease, and liver disease. These heterogeneous diseases have similar characteristic pathological changes, including in situ thrombosis, smooth muscle hypertrophy, and intimal proliferation. Right heart catheterization is essential to confirm diagnosis, determine prognosis, and assign therapy. A minority of patients have a favorable response to an acute vasodilator trial and long-term benefit with calcium channel blocker therapy. Continuous intravenous epoprostenol improves symptoms and survival in patients with advanced primary pulmonary hypertension and has potential benefit in other forms of pulmonary arterial hypertension. Lung transplantation remains an important option for individuals in whom maximal medical therapy fails. The recent discovery of the gene for familial primary pulmonary hypertension and the increase in new drugs undergoing clinical trials are encouraging developments.