Preface Peter M.A. Calverley Welcome to this, the 100th issue of the ERS Monograph. The Monograph is one of the European Respiratory Society’s longest running publications and from its inception it has been a reliable and accessible source of authoritative, up-to-date information about all aspects of respiratory disease. Over the years, we have covered an enormous number of topics, some on several occasions. This revisiting of topics reflects the changes in understanding and progress in therapy that have occurred in our field. In this anniversary issue, we look again at the diagnostic and therapeutic problems raised by pulmonary conditions that are relatively rare but nonetheless very impactful for those who suffer from them. Although they are individually infrequent, taken together, one or more of the wide range of disorders described in this issue is likely to crop up in clinical practice on a regular basis. Thanks to the hard work of our expert Guest Editors, led by Professor Thomas O.F. Wagner, we now have access to an excellent source of information and guidance when caring for these illnesses. This volume is timely, not just because it marks an important milestone in the history of the ERS Monograph but because it allows us to celebrate the success of the European Reference Network for Rare Diseases of the Respiratory System (ERN-LUNG https://ern-lung.eu/), which is itself a lesson in the value of collaboration between experts in many different countries. This is an approach very much in line with the mission of the European Respiratory Society. The contributors writing here are largely drawn from that network and the chapters they have written will engage and inform all who read them. So, we have completed our first 100 issues – now on to the next 100, but not before you have sampled the many interesting topics covered herein. Disclosures: P.M.A. Calverley reports receiving grants, personal fees and non-financial support from pharmaceutical companies that make medicines to treat respiratory disease. This includes reimbursement for educational activities and advisory work, and support to attend meetings. 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. https://doi.org/10.1183/2312508X.10010323 vii