For many thoracic operations lung separation to enable collapse of the operated lung is either necessary or useful for successful surgery. Lung separation may also be required in other settings for management of pulmonary bleeding, infection or complicated lung ventilation. Lung separation requires knowledge, skill and attention to detail. New technology including videolaryngoscopy, tracheal tubes incorporating distal cameras and improvements in bronchial blocker technology have added to the anaesthetist’s armamentarium. This chapter describes the indications, techniques and complications of lung separation of relevance to generalist and specialist anaesthestists.