Our understanding of the pathogenesis and consequences of acute exacerbations of chronic obstructive pulmonary disease (COPD) has increased considerably in the past decade. Several new lines of evidence support bacterial causation of about half the exacerbations. Contrary to previous data, recent studies with improved methodology have demonstrated that exacerbations do contribute to the loss of lung function in COPD. Another interesting new observation is that colonization by bacterial pathogens may not be innocuous, and in fact may lead to airway inflammation and contribute to pathogenesis of COPD. Evidence that bacteria cause exacerbations, that exacerbations contribute to loss of lung function, and that chronic colonization by bacteria may be harmful, has emphasized the potential importance of appropriate antibiotics in the treatment of exacerbations. An unfortunate paucity of data does not allow evidence-based recommendations to be made for optimal choice of antibiotics for exacerbations of COPD. Emerging data that antibiotics differ in unconventional measures of efficacy such as time to next exacerbation, improvement in health-related quality of life, and bacteriologic eradication will help us make concrete recommendations in the future.