Pulmonary arterial hypertension (PAH) is classified within Group 1 of the current PH classification. It is a progressive disease of the small pulmonary arteries, with increasing mean pulmonary vascular resistance eventually leading to right heart failure. Initial diagnosis of PAH can be difficult as patients often present with non-disease-specific symptoms such as breathlessness, fatigue, peripheral oedema, chest pain during exertion, light headedness, syncope and abdominal distension. The disease is heterogeneous: idiopathic PAH is sporadic, without any familial history of PAH or known triggering factor, but PAH can also be heritable, induced by drugs or toxins, or associated with conditions such as connective tissue disease, congenital heart disease, portal hypertension, HIV infection or schistosomiasis. PAH is a rare disease with an estimated prevalence of 7-52 cases per million. Thanks to the introduction of PAH-specific therapies and better disease management, quality of life and survival have improved considerably, but in many cases lung or heart-lung transplant remain the only option for end stage disease. Living with the condition remains challenging and patients have many unmet needs, see the section on Living with PH.