Idiopathic pulmonary arterial hypertension (IPAH) is an isolated small-vessel disease comprising vasoconstriction, remodeling and thrombosis of small pulmonary arteries. However, there is evidence that IPAH does not respect anatomic boundaries and might extend into large vessels such as large central thrombi. On the other hand, chronic thromboembolic pulmonary hypertension (CTEPH) represents a distinct category of pulmonary hypertension as it is thought to be due to an occlusion of the major pulmonary arteries following a thromboembolic event. However, it is currently evident that in most patients there is a concomitant small-vessel disease. The involvement of both small and large vessels in both IPAH and CTEPH, together with a high incidence of silent thromboembolic events, might create difficulties in identifying the true cause of pulmonary hypertension. An accurate diagnosis of the cause determines the management and prognosis. Patients with CTEPH can potentially be offered curative surgery in the form of pulmonary endarterectomy; however, oxygen, vasodilators, anticoagulation, and lung transplantation are more feasible options for IPAH.