ERS | monograph Preface Robert Bals Many lung diseases have a chronic course, and this in itself can be challenging to manage. Diseases like COPD and IPF, for example, slowly deteriorate and treatment is often limited to relieving symptoms. It is disastrous for the patient if the situation gets out of control and the disease gets much worse within a short time. It is often difficult to understand the underlying biological processes of these deteriorations. Exacerbations frequently result in a faster decline of the underlying disease and can cause the death of the patient. The field of exacerbations in pulmonary diseases is complex and inadequately understood, for a number of reasons: 1) Generally, the pathomechanisms of exacerbations are poorly understood. 2) The definitions of exacerbations are often unclear, which can cause additional difficulties in the diagnosis of these sudden deteriorations. 3) It is often difficult to rule out relevant differential diagnoses and in many cases, these seem to be more than one disease entity, including infections. 4) Treatment options can be very limited, which is largely a result of a lack of basic understanding of what is happening. Whilst all of this may seem pessimistic, this area actually provides an opportunity to improve the care of our patients. Exacerbations represent an acute-on-chronic condition and it should be possible to focus future research on mechanisms, diagnosis and treatment. In addition, there is clearly a need to raise awareness about these critical conditions in pulmonary medicine. In comparison with the number of chest pain and stroke units, very few departments handle exacerbations of respiratory diseases in a similar treatment structure, despite the fact that pulmonary exacerbations have a significant impact on the patient and on mortality figures overall. This Monograph provides a comprehensive overview of exacerbations in pulmonary diseases. It covers specific disease entities such as COPD, asthma, CF and IPF, and provides detailed information for the clinician. It also discusses the mechanisms of exacerbation development, which is an important for the prevention of and basic understanding about this area. Copyright ©ERS 2017. Print ISBN: 978-1-84984-089-7. Online ISBN: 978-1-84984-090-3. Print ISSN: 2312-508X. Online ISSN: 2312-5098. https://doi.org/10.1183/2312508X.10012617 ix