Pharmacodynamics of antibacterial agents relates the time course of drug concentration to its antimicrobial effects at the infection site. Antibacterial agents can be divided into three groups based on pharmacodynamic characteristics: agents that exhibit concentration-dependent bactericidal activity over a range of drug concentrations (e.g., aminoglycosides and fluoroquinolones); agents that exhibit time-dependent bactericidal activity that has little relationship to the magnitude of concentration, provided the concentrations are above a minimally effective level (e.g., beta-lactam antibiotics and vancomycin); and agents that exhibit a predominantly bacteriostatic effect. Knowledge of antimicrobial pharmacodynamics provides a rational basis for determining optimal regimens of dosage amounts and length of dosage intervals.