Difficult-to-Treat Severe Asthma Edited by K.F. Chung, E.H. Bel, S.E. Wenzel. E RESPIRAT EUR N U M B E R 5 1 / M A R C H 2 0 1 1 Print ISSN 1025-448x Online ISSN 2075-6674 Print ISBN 978-1-84984-009-5 Online ISBN 978-1-84984-010-1 Difficult-to-treat severe asthma constitutes approximately 5–10% of the asthmatic population and continues to present a considerable therapeutic challenge. Use of the pragmatic definitions of the past 10 years has led to a better delineation of these patients. This issue gathers experts in paediatric and adult severe asthma who have summarised what has been achieved in our understanding of the different phenotypes, their biomarkers and the reasons and mechanisms under- lying difficult-to-treat severe asthma and the refractoriness to current treatments. This issue also provides a physician’s guide to the management of severe asthma with the optimal use of the currently available therapies, while highlighting the potential approaches we need to take in order to discover more effective treatments. EUROPEAN RESPIRATORY monograph Difficult-to-Treat Severe Asthma 51 EUROPEAN RESIATORY EUROPEAN RESPIATORY