Introduction Thomas O.F. Wagner1,2, Marc Humbert 2,3,4,5 ,Marlies Wijsenbeek2,6, Michael Kreuter 2,7 and Helge Hebestreit 2,8 1 Department of Pneumology, University Hospital Frankfurt, Frankfurt, Germany. 2 European Reference Network for Rare Diseases of the Respiratory System (ERN-LUNG), Frankfurt, Germany. 3 Assistance Publique – Hôpitaux de Paris (AP-HP), Dept of Respiratory and Intensive Care Medicine, Pulmonary Hypertension National Referral Center, Hôpital Bicêtre, Le Kremlin-Bicêtre, France. 4 Université Paris-Saclay, Faculty of Medicine, Le Kremlin-Bicêtre, France. 5 INSERM UMR_S 999 “Pulmonary Hypertension: Pathophysiology and Novel Therapies”, Le Kremlin-Bicêtre, France. 6 Center of Excellence for Interstitial Lung Diseases and Sarcoidosis, Department of Respiratory Medicine, Erasmus Medical Center-University Medical Center Rotterdam, Rotterdam, The Netherlands. 7 Mainz Center for Pulmonary Medicine, Department of Pneumology, Mainz University Medical Center and Department of Pulmonary, Critical Care &Sleep Medicine, Marienhaus Clinic Mainz, Mainz, Germany. 8 Pediatric Pulmonology &Cystic Fibrosis, Children’s Hospital, University Hospital Würzburg, Würzburg, Germany. Corresponding author: T.O.F. Wagner (t.wagner@em.uni-frankfurt.de) @ERSpublications Rare respiratory diseases pose a significant burden and can be challenging to diagnose and treat. This Monograph provides an up-to-date, comprehensive resource to the clinician, both for educational purposes and for clinical care. https://bit.ly/ERSM100intro Copyright ©ERS 2023. Print ISBN: 978-1-84984-166-5. Online ISBN: 978-1-84984-167-2. Print ISSN: 2312-508X. Online ISSN: 2312-5098. Respiratory system disorders play a crucial role in the burden of disease and account for a huge portion of morbidity and mortality. People with rare diseases – according to the European definition, affecting not more than five in 10 000 – share symptoms and functional impairments with many more common diseases. Diagnosing these diseases can be challenging due to their rarity, and treating these conditions is complex because of often quite specific needs and treatment options. To address this, the European Respiratory Society (ERS) has published Rare Diseases of the Respiratory System – the 100th issue of the ERS Monograph. The previous Monograph in this thematic area, entitled Orphan Lung Diseases and edited by Jean-Francois Cordier [1], was published in 2011 and needed an update. To reflect the close collaboration of ERS with the European Reference Network for Rare Diseases of the respiratory system (ERN-LUNG https://ern-lung.eu/), Thomas O.F. Wagner, was honoured to guest edit this new Monograph within a team of esteemed co-guest editors, since dealing with rare diseases nowadays requires networking and teamworking. This collaboration of networks has allowed for most of the recent progress made. A good example of the impact of networking on the improvement of care and research for people with rare lung diseases are the clinical trials networks. These collaborative infrastructures have fostered the development and market authorisation of many new drugs for rare diseases. Another evolutional change of this book compared with its predecessor is that the reviews it contains now offer a more comprehensive overview of the whole spectrum of rare diseases of https://doi.org/10.1183/2312508X.10009923 xi