Investigations over the past 20 years have demonstrated that antibacterials can vary markedly in the time course of antimicrobial activity. These differences in pharmacodynamic activity have implications for optimal dosage regimens. The results of more recent studies suggest that the magnitude of the pharmacokinetic/pharmacodynamic parameters required for efficacy are relatively similar in animal infection models and in human infections. However, there is still much to learn. Additional studies are needed to further correlate pharmacokinetic/pharmacodynamic parameters for many antibacterials with therapeutic efficacy in a variety of animal infection models and in human infections. The potential value of using pharmacokinetic/pharmacodynamic parameters as guides for establishing optimal dosing regimens for new and old drugs and for new emerging pathogens and resistant organisms, for setting susceptibility breakpoints, and for reducing the cost of drug development should make the continuing search for the therapeutic rationale of antibacterial dosing of mice and men worthwhile.