Pulmonary hypertension is characterised by elevated pulmonary artery pressures. This chapter incorporates a systematic approach to the condition. By discussing the pathophysiology of the disease, it builds a structure for understanding the diverse diseases that may result in PH through differing mechanisms. The pulmonary vascular circuit is described, the basis of pressure measurement within its components explained, and the relevance of application of the pre- and post-capillary PH concept to the ESC/ERS classification examined. The common scenarios wherein the pulmonologist encounters PH are discussed, along with the different diagnostic modalities used in its diagnosis and quantification. A practical strategy of systematically isolating the cause for PH is also discussed.