ERS | monograph Preface Robert Bals (outgoing Editor in Chief) and John R. Hurst (incoming Editor in Chief) The concept of microbiome research is a hot topic and multiple diseases have been linked with a dysbiosis at various sites of the body. The lay press has featured many aspects of this topic and attracted attention to this research. The basic techniques of this field have been developed outside the medical area and originate in terrestrial microbiology. The methodologies have developed rapidly and are closely linked with NGS, high-throughput analysis, personalised medicine and bioinformatic analysis of big data. While most papers in this area address the microbiome of the gut, the field of microbiome research in pulmonary medicine has also attracted a lot of activity. The microbiome of the lung is a fascinating area because it allows us to integrate a novel view on the coexistence of the body and micro-organisms into mechanisms of disease development. The lung must have a low abundance of micro-organisms and only minor inflammation in order to function adequately. Strong host defence instruments quickly eliminate micro-organisms. Genetic and environmental harm cause a breach in the pulmonary host defence, with subsequent dysbiosis. There are reports that describe alterations of the microbiota in almost all lung disease entities. Nevertheless, it is often unclear whether these changes have a role in disease development or whether they are a secondary result of the underlying disease. This Monograph provides the reader with a broad and detailed overview of various aspects of the lung microbiome. The Guest Editors, Michael J. Cox, Markus J. Ege and Erika von Mutius, have brought together chapters that consider methodology and ontogenesis, as well as disease-specific chapters, on asthma, COPD, CF, interstitial lung disease and pneumonia. I thank the Guest Editors for their work together with the authors of the individual chapters, they have produced a unique and comprehensive introduction to the field. I am sure this book will help develop understanding about the concepts and application of this novel area of research. Copyright ©ERS 2019. Print ISBN: 978-1-84984-101-6. Online ISBN: 978-1-84984-102-3. Print ISSN: 2312-508X. Online ISSN: 2312-5098. Robert Bals John R. Hurst https://doi.org/10.1183/2312508X.10013019 v