Rare lung diseases are one of the most exciting topics in respiratory medicine. However, as these are infrequent diseases evidence-based guidelines do not exist for most of the different entities. Nevertheless, progress has been made during the last decade either in diagnostics or therapy, which has greatly changed clinical practice. This issue of the European Respiratory Monograph, with contributions from well-known experts in the field, summarises the current knowledge of orphan lung disease in one edition for the first time. This allows respiratory physicians, either in the clinic or in research, to obtain an overview about the whole field of these diseases.