The term haemoptysis derives from two ancient Greek words, haima (blood) and ptysis (spitting out) (Blasi, Pathophysiology and causes of haemoptysis. In: Webb A, Angus D, Finfer S, Gattinoni L, Siger M (eds) Oxford textbook of critical care (2nd edn). © Oxford University Press, 2016). In the strict sense of the word, an upper respiratory or a gastrointestinal bleed can both produce hemoptysis, that is to say, a spitting out of blood. However, in clinical parlance, hemoptysis refers to bleeding from the lower respiratory tract—in other words, the expectoration of blood in sputum (sputum being the fluid that originates in the lower respiratory tract). In this chapter, a discussion on the pathophysiology of hemoptysis is followed by an appraisal of the various oetiological possibilities involved, and a comparative analysis of the modalities used in their evaluation.