ERS | monograph Introduction David J. Jackson1,2, and Michael E. Wechsler3 @ERSpublications This Monograph covers eosinophilic lung diseases, a long-standing challenge to clinicians. Recent research has increased understanding of eosinophil biology and has led to new therapeutic options, helping patients with these difficult-to-control diseases. https://bit.ly/3mcgc21 While eosinophils were discovered in the late 1800s, it has really only been over the last 20 years that those who treat lung disease have gained an appreciation of their importance in both health and disease. It has long been recognised that eosinophils play a role in homeostasis, defence against helminths and other infections, and are even involved in tumour immunity. However, it is their role in a variety of different diseases of the lung and other systemic disorders that has challenged clinicians for several decades. It has long been recognised that eosinophilic conditions like eosinophilic asthma, CEP, EGPA and other HESs are associated with significant morbidity and are difficult to treat. Corticosteroids have long been the mainstay of therapy and, along with other immunosuppressants, have been associated with significant morbidity in and of themselves. As we have gained a better understanding of eosinophil biology, we have also gained a better understanding of eosinophilic disorders, and over the last decade, we have finally developed effective therapies that target the eosinophil, and for the first time, are approved for treatment of these entities, revolutionising the care of our patients. It is in this context that we set out to develop this ERS Monograph. So much knowledge has been gained in the last few years but unanswered questions concerning eosinophil biology still remain, including whether there are any long-term consequences of blocking eosinophil activity. The overarching goal of this Monograph is to take the reader on a journey through the different eosinophilic lung diseases so that the practicing clinician is better equipped to recognise and treat patients with eosinophilia. First, however, we step back and review eosinophil biology so that the underpinnings of eosinophilic diseases and their targets can be better understood [1]. While primary eosinophilic disorders of the lung are a major focus of this Monograph, we recognise that there are several eosinophilic disorders that need to be excluded. From infections through malignancies to drug reactions, the chapter on the differential diagnosis Copyright ©ERS 2022. Print ISBN: 978-1-84984-142-9. Online ISBN: 978-1-84984-143-6. Print ISSN: 2312-508X. Online ISSN: 2312-5098. 1 Guy’s Severe Asthma Centre, Guy’s and St Thomas’ NHS Foundation Trust, London, UK. 2 Faculty of Life Sciences and Medicine, King’s College London, London, UK. 3 National Jewish Health, Denver, CO, USA Correspondence: David J. Jackson (david.jackson@gstt.nhs.uk) viii https://doi.org/10.1183/2312508X.10021521