Pulmonary arterial hypertension (PAH) is characterised by progressive remodelling in the distal pulmonary arteries leading to increased pulmonary vascular resistance and thus right ventricular afterload. This leads ultimately to right ventricular failure and death. The traditional view of chronic thromboembolic pulmonary hypertension (CTEPH) is that is results following an episode of pulmonary embolism (PE) which fails to resolve. However, we know that CTEPH is also a progressive condition even when further PEs arebeing prevented with anticoagulation. This progression most likely comes from a progressive pulmonary vascular remodelling process, which in many ways is indistinguishable from the changes seen microscopically in PAH.