Patients with chronic obstructive pulmonary disease (COPD) are especially prone to bronchial colonization and acute infection, including acute infectious exacerbations and community-acquired as well as nosocomial pneumonia. Bacterial colonization may promote bronchial inflammation, predispose to acute exacerbations, and thereby contribute to respiratory decline.Haemophilus influenzaeandStreptococcus pneumoniaeare the most frequently encountered pathogens in both stable disease as well as acute exacerbations. The role of bacterial infection in acute exacerbations remains subject to controversy; however, evidence is in favor of regular antimicrobial treatment in patients with severe exacerbations. COPD represents an important risk factor for community-acquired pneumonia (CAP). Microbial patterns for CAP differ from exacerbations in that S.pneumoniaeand “atypical” bacterial pathogens predominate. Similarly, COPD has been recognized as an important risk factor for nosocomial pneumonia. Initial empiric antimicrobial treatment of both severe acute exacerbations and pneumonia should take into account regional drug resistance rates to penicillin ofS. pneumoniaeand should cover Gram-negative enteric bacilli. The additional treatment of atypical bacterial pathogens andPseudomonas aeruginosashould be considered individually.