Chronic thromboembolic pulmonary hypertension (CTEPH) is classified as group 4 pulmonary hypertension (PH) by the World Health Organization. It is a rare, but underdiagnosed, complication of acute pulmonary embolism (PE) resulting from occlusion of large pulmonary arteries with a fibrothrombotic material and in many cases, the development of a distal vessel arteriopathy that closely mimics pulmonary arterial hypertension. While the pathogenesis of CTEPH is not entirely understood, the primary pathophysiology appears to be related to an initial thrombotic event followed by both inadequate thrombus resolution and secondary arteriopathy. Several risk factors for development of CTEPH have been found, including the size of the initial PE, splenectomy, chronic inflammatory conditions, and indwelling catheters and cardiac pacemaker leads. There is however an increased incidence of elevated factor VIII levels in CTEPH, and this finding persists after treatment. However, none of these findings explain the development of CTEPH in the majority of patients affected by this disease.