This article reviews pharmacodynamics of antibacterial drugs, which can be used to optimize treatment strategies, prevent emergence of resistance and rationalize the determination of antimicrobial susceptibility. Important pharmacodynamic concepts include the requirements for bactericidal therapy for endocarditis and meningitis, for synergistic combinations to treat enterococcal endocarditis or to shorten the course of antimicrobial therapy, for obtaining maximal plasma concentration/minimal inhibitory concentration (MIC) ratios that are greater than 10 or 24 hour-area under the plasma concentration curve (AUC)/MIC ratios that are greater than 100-125 for concentration-dependent agents against gram-negative bacilli and 25-35 against Streptococcus pneumoniae, and for obtaining percent of time that drug levels are greater than the MIC that is at least 40% to 50% of the dosing interval for time-dependent agents.