Acute exacerbations of COPD are a major cause of morbidity and mortality. Bacteria are implicated in about half of all cases. The frequency of exacerbations is related to decline in lung function and poorer quality of life. 25% of patients with COPD have bacterial colonization of the lower airways in stable state whereas non-smokers without COPD have airways that are sterile. The significance of the colonization is unclear, but there is emerging evidence that it may be detrimental. Much of the data recommending antibiotic treatment are based on findings more than 10 years old and do not take into account emerging bacterial resistance. This article reviews these data and that from newer antibiotic trials. It also reviews current antibiotic prescribing guidelines from major respiratory societies around the world. Recent antibiotic trials have compared fluoroquinolones with "standard" antibiotics and found, in the main, longer exacerbation-free intervals and better bacterial eradication rates in those treated with fluoroquinolones.