Resistance of nosocomial and community‐acquired pathogens to antimicrobial agents is a serious problem with significant clinical consequences. Microbiologic surveillance data, such as those provided by the National Nosocomial Infections Surveillance System, supply information on current nosocomial pathogens in the United States. Many species show resistance to commonly used antimicrobials and, in many cases, it is emerging resistance. Resistance in many gram‐negative bacteria is caused by β‐lactamase production.Escherichia coli, the leading nosocomial pathogen, is capable of hyperproducing TEM‐1 β‐lactamase. A novel form of resistance inKlebsiella pneumoniaeandE. coliis caused by extended‐spectrum cephalosporinases. Many Enterobacteriaceae can be induced to produce group 1 β‐lactamase by exposure to broad‐spectrum cephalosporins and other β‐lactams. Thirty percent ofHaemophilus influenzaeisolates are resistant to ampicillin because of β‐lactamase production. Issues of concern in gram‐positive species include multiple antimicrobial resistance in methicillin‐resistantStaphylococcus aureus, enterococci, and coagulase‐negative staphylococci, and increasing β‐lactam resistance inStreptococcus pneumoniae.To minimize the development of resistance, antimicrobials must be administered judiciously, and infection‐control practi