The gram-positive bacterium Staphylococcus aureus is a leading cause of hospital- and community-associated infections (16, 85, 108). In the hospital, S. aureus is the most frequent cause of surgical, lower respiratory tract, and cardiovascular infections. Furthermore, it is the second most common cause of health care-associated pneumonia and bloodstream infections (108, 151, 152). Historically, --lactam antibiotics have exhibited potent activity against S. aureus, which along with good safety profiles make them the agents of choice for the treatment of staphyloccocal infections. Of particular concern now is the growing prevalence of methicillin (meticillin)-resistant S. aureus (MRSA) in both hospital- and community-associated infections (24, 70, 133). The development of resistance to --lactam antimicrobials, often concurrently with resistance to other antimicrobial agents, poses a great challenge to the prevention and treatment of S. aureus infections (7, 108). Staphylococci have two primary mechanisms for resistance to --lactam antibiotics: the expression of an enzyme (the PC1 --lactamase) capable of hydrolyzing the --lactam ring, thus rendering the antibiotic inactive, and the acquisition of a gene encoding a modified penicillin-binding protein (PBP), known as PBP 2a, found in MRSA and coagulase-negative staphylococci. PBP 2a is intrinsically resistant to inhibition by --lactams (59). PBP 2a remains active in the presence of concentrations of --lactam antibiotics that inhibit most endogenous PBP enzymes, thus substituting for their functions in cell wall synthesis and allowing growth in the presence of the --lactam inhibitors. This review briefly discusses the structure and synthesis of the S. aureus cell wall, the resistance to --lactam antibiotics through the acquisition of PBP 2a, the evolution of MRSA, and the involvement of other protein factors in methicillin resistance. In addition, the characteristics of new --lactam antibiotics that target PBP 2a are discussed, along with their role as important new entities in the antibacterial pipeline for the treatment of MRSA infections.