The European Respiratory Society (ERS)/American Thoracic Society (ATS) Task Force on severe asthma includes an updated definition of severe asthma, a discussion of severe asthma phenotypes in relation to genetics, natural history, pathobiology and physiology, as well as sections on evaluation and treatment of severe asthma where specific recommendations for practice are made. See the unabridged online version of the document for detailed discussion of the definition of severe asthma, phenotypes and recommendations for practice.