Emergence of resistance to antimicrobial agents among previously susceptible organisms continues to be an important obstacle to the successful treatment of bacterial infections. In hospitals, plasmid-mediated resistance to third-generation cephalosporins and monobactams has recently appeared in gram-negative bacilli, due primarily to mutations in TEM- and SHV-type enzymes. Among nosocomial enterococci, vancomycin resistance, beta-lactamase production, and high-level resistance to all aminoglycosides have recently been added to this organism's already formidable armamentarium of resistance properties. Also, resistance to fluoroquinolones and rifampin has been emerging in methicillin-resistant Staphylococcus aureus. In the community, organisms in which resistance plays a particularly important role are shigellae, Haemophilus influenzae, gonococci, and pneumococci, particularly in developing countries. beta-lactamase-producing meningococci have been reported for the first time. The selective pressure generated by the use of antimicrobial agents, together with the ability of bacteria to acquire and spread resistance and the capacity of humans to transmit bacteria, suggest that antimicrobial resistance will continue to be a problem for the foreseeable future.