xv Preface Respiratory sleep medicine is a rapidly evolving discipline in pneumology. Since the first edition of the ERS Handbook of Respiratory Sleep Medicine, we have seen significant progress in the pathophysiological understanding of the various endotypes of obstructive sleep apnoea, described distinct phenotypes based on symptoms and comorbidities, and gained insights into the limitations and potential of biomarkers. This helps us to reformulate a pure mechanistic understanding of the disease. Moreover, we have proceeded from a generic definition of the disease based on the apnoea–hypopnoea index to its replacement by outcome-oriented or patient-related biomarkers. Similarly, we are currently discovering important information about the different phenotypes of central sleep apnoea and its optimal, personalised treatment. Finally, large randomised controlled studies have produced unexpected results, that underline the urgent need for a change in study design and use of refined statistical analysis based on large number of patients. Therefore, an update of the ERS Handbook of Respiratory Sleep Medicine is clearly necessary. Sleep medicine is a true multidisciplinary field. Sleep physicians are referred and treat patients from all specialties of medicine. Therefore, we are grateful for the contributions from colleagues, not only from pulmonary medicine, but also from neurology, paediatrics, psychiatry and ENT, among others. We have worked to ensure the book provides a valuable update, not only for experienced sleep specialists, but also for trainees, nurses and allied healthcare professionals. Our aim is to focus on practical aspects, tips and advice based on clinical practice and up-to-date guidelines. We are really grateful to everyone who contributed to this edition. Maria R. Bonsignore, Winfried Randerath, Sophia E. Schiza and Anita K. Simonds Chief editors