Much of the morbidity and mortality in chronic obstructive pulmonary disease relates to symptomatic deteriorations in respiratory health termed exacerbations. Exacerbations also are associated with changes in lung function and both airway and systemic inflammation. The most common causes of exacerbation are micro-organisms: respiratory viruses such as rhinovirus, and various bacterial species. This article reviews and discusses current understanding of the biology of exacerbations, considering the definition, epidemiology, etiology, and the nature and evolution of the changes in symptoms, lung function, and inflammation that characterize these important events.