Pulmonary hypertension has traditionally been classified as primary with no identifiable cause or secondary to an identifiable cause. It has been noted that some diseases classified as secondary pulmonary hypertension resemble primary pulmonary hypertension in their histopathologic features and response to therapy. The 2003 World Symposium on Pulmonary Arterial Hypertension in Venice, Italy, revised the Second World Symposium on Pulmonary Hypertension or Evian Classification to produce the current World Health Organization (WHO) classification scheme for pulmonary hypertension. This scheme classifies pulmonary hypertension in five groups based on mechanisms instead of associated conditions: (i) pulmonary arterial hypertension (PAH), (ii) pulmonary hypertension with left heart disease, (iii) pulmonary hypertension associated with lung diseases and/or hypoxemia, (iv) pulmonary hypertension due to chronic thrombotic and/or embolic disease, and (v) miscellaneous (including sarcoidosis, histiocytosis X, lymphangiomatosis, and compression of pulmonary vessels). In the revision of the WHO classification in Venice in 2003, which discarded the terms primary and secondary pulmonary hypertension, pulmonary arterial hypertension was classified as (i) idiopathic (essentially composed of conditions in the previous category of primary pulmonary hypertension), (ii) familial, (iii) associated with other conditions such as collagen vascular diseases or congenital systemic to pulmonary shunts, (iv) pulmonary hypertension associated with significant venous or capillary involvement (pulmonary veno-occlusive disease and pulmonary capillary hemangiomatosis), and (v) persistent pulmonary hypertension of the newborn. Specific diagnoses are discussed in the chapters in Section 10. In addition, specific types of emboli are covered in this section, including those that may contribute to pulmonary hypertension.