Hypersensitivity pneumonitis (HP) is an immune-mediated reaction of the lung that is provoked by inhaled antigens. Firm diagnostic criteria are currently lacking and so the diagnosis of HP depends upon the integration of history, imaging and, where necessary, lung biopsy. This chapter adopts a methodical approach to HP. It discusses the drawbacks of traditional classification systems and explains how recent classification systems attempt to address those drawbacks. It also enumerates the possible antigens to seek, where significant exposures occur. It describes the various clinical and imageological aspects that characterise acute and chronic HP, and help in their discrimination from related conditions. Also presented in the chapter is a discussion on the utility of the supportive tests that help in diagnosis; and the role of broncho-alveolar lavage (BAL) and biopsy in diagnosis, with the attendant caveats.