Acute exacerbations of chronic obstructive pulmonary disease (AECOPD) are episodes of symptom worsening which have significant adverse consequences for patients. Exacerbations are highly heterogeneous events associated with increased airway and systemic inflammation and physiological changes. The frequency of exacerbations is associated with accelerated lung function decline, quality of life impairment and increased mortality. They are triggered predominantly by respiratory viruses and bacteria, which infect the lower airway and increase airway inflammation. A proportion of patients appear to be more susceptible to exacerbations, with poorer quality of life and more aggressive disease progression than those who have infrequent exacerbations. Exacerbations also contribute significantly to healthcare expenditure. Prevention and mitigation of exacerbations are therefore key goals of COPD management.