COPD is the third leading cause of death worldwide and a major cause of morbidity. Infection complicates COPD, both when patients are stable and during exacerbations. Exacerbations drive disease progression, lung function decline and premature death, therefore understanding and targeting airway infection are considered crucial. The interactions of bacteria, viruses and airway inflammation have all been shown to be a factor in stable COPD and in exacerbations. New molecular microbiology techniques are changing our view of COPD, identifying new bacterial species and a diverse microbiome which can be disturbed in chronic lung disease. Bronchiectasis is increasingly recognized in COPD, and may be a distinct sub-type or "phenotype" of disease associated with greater bacterial colonization and inflammation. Increasingly, severe infections, and particularly pneumonia are recognized as common in COPD, and the role of inhaled corticosteroids in provoking bacterial colonization and development of pneumonia requires careful consideration. Long term antibiotic therapies are being explored as a means to prevent exacerbations and disease progression in COPD. This paper brings together the current knowledge related to the role of infection in COPD.