Knowledge of the antimicrobial pharmacodynamic characteristics of a drug (inhibition of growth, rate and extent of bactericidal action, and postantibiotic effect) provides a more rational basis for determination of optimal dosing regimens in terms of the dose and the dosing interval. This article reviews concepts of antimicrobial pharmacodynamics, the effect of pharmacodynamics on the emergence of resistant bacterial subpopulations, and the development of pharmacodynamic breakpoints for use in the design of trials of these drugs and in treatment of infected patients.