Anaerobic bacteria are relatively common and important pathogens in the lower airways, but are rarely detected due to problems in obtaining adequate specimens for anaerobic bacterial culture. As a consequence, therapeutic decisions are generally empiric and made on the basis of suspected involvement of these organisms according to various clinical clues. With respect to antibiotic options, there is probably a multitude of drugs that would be effective, but there are only three that have a sufficient experience according to published reports: clindamycin, penicillin, and metronidazole combined with penicillin. Recent studies suggest that many of the bacteria involved in these infections produce beta-lactamase, consequently favoring drugs other than penicillins for these infections. Nevertheless, the in vivo experience continues to be relatively good for penicillin when given for orodental or pulmonary infections involving anaerobes derived from the upper airways. For a serious anaerobic infection, such as putrid abscesses associated with large cavities or severe toxicity, the usual drug recommendation is clindamycin. For less serious infections, regimens with established merit are noted above. It is likely that almost any beta-lactam (other than antistaphylococcal penicillins, ceftazidime, or azthreonam) would be adequate; metronidazole should not be used as a single agent.