The diagnosis and treatment of acute exacerbations (AEs) of chronic obstructive pulmonary disease (COPD) is controversial. In this section, we review (1) the epidemiology of this condition; (2) the etiology--many patients with AECOPD are thought to have a combination of viral and bacterial infections, which contribute to their exacerbation. Bacterial organisms are isolated more commonly after viral infections in patients with COPD. The role that bacterial infections play in AECOPD remains a very controversial topic; (3) the use of diagnostic procedures; (4) efficacy of antibiotics; (5) clinical parameters to stratify patients' severity; (6) different groups of antibiotics that can be used; and (7) other therapies, including bronchodilators. We summarize the current literature, with special emphasis on the assessment of the long-term impact of this condition.