Preface Tremendous progress has been made in the treatment of patients with asthma during the past decades. Whereas 30 years ago patients suffering with acute asthma attacks were admitted to the emergency department several times a day, today this has been reduced to a couple of patients a year. Treatment with inhaled corticosteroids (ICS) in the first instance, followed by the use of combination therapy with ICS and long-acting b-agonists later on has greatly improved the long-term prognosis of asthma patients. Nevertheless, there are a number of patients that are not well controlled even with a high-dose combination therapy. Near fatal asthma attacks still occur and asthma mortality, although low, is still present. Difficult-to-treat asthma has been recognised as the severe form of the ‘‘other’’ asthma entities in the past. However, the understanding of the pathophysiology of this kind of asthma has been better understood during the last few years. Different phenotypes of difficult-to-treat asthma have been recognised and have led to new treatment strategies focused on pathophysiological concepts and individual clinical signs and symptoms of the patient. More and more insights into cell and molecular biology, genomics, epigenetics, epidemiology and clinical presentation of the asthma patients make it possible to start with a more personalised treatment strategy that orientates itself on the patients needs. The Guest Editors of this issue have succeeded in attracting leading experts from within the field to write chapters and provide an overview from current pathophysiology, diagnostic strategies and treatments to future developments and what is perceived to be on the horizon. I would like to congratulate the Guest Editors for this excellent Monograph, which will be of great interest to both basic scientists and clinicians, who will find it both insightful and resourceful in their daily practice. Editor in Chief T. Welte Eur Respir Mon 2011. 51, vi. Printed in UK – all rights reserved, Copyright ERS 2011. European Respiratory Monograph ISSN: 1025-448x. DOI: 10.1183/1025448x.10003411 vi