Although strains of Staphylococcus aureus resistant to penicillin have caused infections for many years, isolates resistant to methicillin, oxacillin, and other ß-lactams have become predominant-primarily in the last 20 years. Strains resistant to ß-lactams and other cell-wall-active agents fall into several categories. Methicillin-resistant S. aureus (MRSA) strains were first described in England in 1961, shortly after methicillin became available for clinical use. They have subsequently spread throughout the world and are an important cause of nosocomial infections in many geographic areas, including the United States. Data from the National Nosocomial Infection Surveillance System reveal MRSA accounts for up to 40% of nosocomial S. aureus infections in large hospitals and 25% to 30% of such infections in smaller hospitals. The vast majority of MRSA infections are acquired in hospitals or long-term care facilities (LTCFs). In a few cities, MRSA has been acquired in the community by intravenous drug users. Several recent reports have suggested MRSA may occasionally be transmitted in other community settings, particularly among preschool-age children, some of whom have attended daycare centers. Further studies are needed to determine if transmission of MRSA in community settings is becoming more common or is limited to a few geographic areas.