Pulmonary hypertension (PH) is defined by a systolic pulmonary artery pressure exceeding 32 (or up to 45) mmHg, or a diastolic pressure that exceeds 20 mmHg. The most recent clinical classification scheme for PH in human medicine utilizes pathophysiological characteristics and response to therapy. This classification replaces the previous designation of primary versus secondary PH. Application of the new classification to two retrospective studies in dogs reveals that the most common is PH with left heart disease, while the second most common is PH associated with lung diseases and/or hypoxaemia. Primary (idiopathic) pulmonary arterial hypertension has been reported in dogs but is the least likely. PH as a complication of cardiopulmonary disease seems to be reported more commonly in dogs than in cats. Physiology; History and physical examination; Diagnosis; Treatment; and Prognosis are all addressed in this chapter.