β-Lactam antibiotics are critical agents in the treatment of anaerobic infections. Susceptibility to these agents, however, varies widely, depending on the specific drug and organism; has not been constant over time; and differs in various geographic locations within the United States for many species. For the organisms in the Bacteroides fragilis group, the β-lactam antibiotics with the most consistent activity are imipenem and combinations of a β-lactam drug plus a β-lactamase inhibitor, such as ticarcillin/clavulanate and ampicillin/sulbactam. Antibiotics with less activity include cefoxitin, piperacillin, cefotetan, and ceftizoxime. In other species of anaerobic gram-negative bacilli, β-lactamase production is seen with increasing frequency. In vitro susceptibility of these strains is now similar to that of the B. fragilis group, with imipenem, ticarcillin/clavulanate, ampicillin/sulbactam, and cefoxitin being the most active drugs. The anaerobic gram-positive cocci and bacilli remain, for the most part, highly susceptible to penicillins and imipenem.