A pleural effusion is the accumulation of fluid in the pleural space. It is a relatively common finding in clinical practice. The diagnostic approach to the patient presenting with a pleural effusion is aimed at defining the effusion as a transudate or an exudate. This review summarises the initial assessment and investigation of pleural effusions diagnosed during the acute medical take, and who should be referred for specialist advice. In addition, recent developments, including the measurement of NT-proBNP levels and diagnostic markers for mesothelioma, are presented.