Introduction Francesco Bonella 1 , Daniel A. Culver2 and Dominique Israël-Biet3 1 Center for Interstitial and Rare Lung Diseases, Pneumology Dept, Ruhrlandklinik University Hospital, University of Duisburg-Essen, Essen, Germany. 2 Dept of Pulmonary Medicine, Respiratory Institute, Cleveland Clinic, Cleveland, OH, USA. 3 Université de Paris and Center of rare pulmonary diseases, Hôpital Européen Georges Pompidou, Assistance Publique-Hôpitaux de Paris, Paris, France. Corresponding author: Francesco Bonella (Francesco.Bonella@rlk.uk-essen.de) @ERSpublications This book includes the voices of clinicians, basic scientists and patients to better illustrate the most recent advances in research and care of sarcoidosis, a many-faceted disease with complex pathogenesis and various clinical manifestations https://bit.ly/32cyVUo Copyright ©ERS 2022. Print ISBN: 978-1-84984-145-0. Online ISBN: 978-1-84984-146-7. Print ISSN: 2312-508X. Online ISSN: 2312-5098. Sarcoidosis is a systemic, multi-pathway disease and a clinical chameleon. Recent reports indicate that the prevalence of sarcoidosis is rising and the mortality in patients suffering from chronic sarcoidosis is increasing [1]. The high variability of manifestations, from asymptomatic to life-threatening, depends on organ involvement and disease activity, which still lacks a clear definition. About half of patients experience a chronic course. The consequences on quality of life and wellbeing can be devastating, especially if patients are not promptly referred to sarcoidosis specialists. To date, there are few approved treatments for sarcoidosis, mostly based on expert opinion, and the majority of investigational drugs have failed in clinical development. Although enormous advances have been made in the understanding of the disease pathogenesis and in the development of new diagnostic tools and management strategies, the burden of sarcoidosis, both socially and economically, is still considerable and the unmet needs persist. It has been 17 years since the first ERS Monograph on Sarcoidosis, edited by Marjolein Drent and Ulrich Costabel, was published [2]. This Monograph is therefore timely and our main aim is to provide the reader with a comprehensive overview of the most recent advances in sarcoidosis. We begin the book by illustrating the newest data on epidemiology [3] and cover the evidence for pathobiology of granuloma formation, including aetiological agents [4], the link between genotypes and phenotypes, and the genetic mutations occurring in familiar forms [5]. We then move on to discuss phenotyping, specific organ manifestations [6–9] and general diagnostic pathways [10], from conventional radiographic features to the use of novel modalities (e.g. PET scan) for diagnosis and assessment of disease activity [11]. We will also provide an update on the usefulness of circulating and imaging biomarkers for assessing disease severity and treatment response [12]. Traditional as well as innovative treatment strategies will be discussed, providing insight into unique aspects of therapy that differ between various organs. In two chapters focusing on when x https://doi.org/10.1183/2312508X.10000622