In patients with COPD, an acute worsening of respiratory symptoms is often described as an exacerbation. Exacerbations are associated with a significant increase in mortality, hospitalization, and health-care utilization, but there is currently no widely accepted definition of what constitutes an exacerbation of COPD. This paper summarizes the discussions of the workshop, "COPD: Working Towards a Greater Understanding," in which the participants proposed the following working definition of an exacerbation of COPD: a sustained worsening of the patient's condition, from the stable state and beyond normal day-to-day variations, that is acute in onset and necessitates a change in regular medication in a patient with underlying COPD.