Pulmonary arterial hypertension (PAH) is defined as a group of diseases characterized by a progressive increase of pulmonary vascular resistance leading to right ventricular failure. It includes a variety of pulmonary hypertensive diseases with different etiologies but similar clinical presentation. PAH is a disease of the small pulmonary arteries, characterized by progressive obliteration of the pulmonary vascular bed. Vasoconstriction, remodeling of the pulmonary vessel wall and thrombosis contribute to an increased pulmonary vascular resistance. Major advances in our understanding of the mechanism of disease development have been achieved over the past decade. Several of these new insights have led to the development and clinical application of novel treatments that includes new classes of drugs such as prostanoids, endothelin receptor antagonists and type 5 phosphodiesterase inhibitors.