The hepatopulmonary syndrome (HPS) is a condition characterised by pulmonary gas exchange abnormalities and widespread intrapulmonary vascular dilatation in the presence of advanced chronic liver disease. This chapter introduces the principles of diagnosis of HPS and discusses the pathophysiological mechanisms of the chronic liver diseases associated with it. It also discusses the pathogenesis of HPS and the impact the development of HPS has on the background liver condition. It outlines the clinical findings in a patient with chronic liver disease that may be considered markers for its development and outlines the special complications that the clinician should be on the lookout for. The chapter also explores the pathophysiology and characteristics of hypoxaemia, the measures for quantification of the associated oxygenating defect, and the methods that can be used for the diagnosis of an intrapulmonary shunt in the setting of HPS.