The incidence of pulmonary thromboembolism is hard to assess by the pathologist as a result of seasonal variation of embolism and disappearance of emboli by thrombolysis. However, the great differences in estimates of the incidence in routine hospital autopsies is mainly related to variation in scrutiny of the investigation and in size of area searched microscopically. Obstruction of major pulmonary arteries almost always results from thromboembolism which is most often found in its acute stage. In chronic major vessel embolism, arterial obstruction by an organized mass may produce pulmonary hypertension. Recanalization of such a mass results in so-called bands and webs. There are no reliable criteria to differentiate between emboli and primary thrombi or their sequelae. In peripheral, particularly muscular pulmonary arteries, thrombi are most likely primary, especially when associated with advanced age and with pulmonary hypertension. However, small arteries may be subject to extensive microembolism following fragmentation of large thromboemboli. Thrombotic arteriopathy is the pulmonary arterial disease based upon either primary thrombosis or embolism. It is often associated with pulmonary hypertension, and characterized by irregular, nonlaminar, often obliterative, intimal fibrosis. Recanalization channels, sometimes widening to separate intravascular fibrous septa, are characteristic features. Reversibility of post-thrombotic lesions is very limited.