Three pharmacokinetic/pharmacodynamic parameters--(i) the peak concentration to the minimum inhibitory concentration ratio (C(max)/MIC); (ii) the area under the concentration-time curve to MIC ratio (AUC(24h)/MIC); and (iii) the time the concentration exceeds the MIC (T>MIC)--are important predictors of the clinical efficacy of antibiotics. For antibiotics with pronounced concentration-dependent killing, such as the fluoroquinolones or the aminoglycosides, C(max)/MIC and AUC(24)/MIC are the main factors that establish efficacy. Antibiotics with a weak, or no, concentration dependency generally have their efficacy linked to T>MIC, and these include the beta-lactams and the conventional macrolides. Antibiotics with weak concentration-dependent effects, but with prolonged persistent effects, such as tetracyclines and azithromycin, have their activity mostly related to the AUC(24)/MIC. By applying these concepts to current antibiotics, and also to the development of novel agents, it is possible to optimise their dosages and administration schedules. This will maximise therapeutic efficacy, may prevent or delay the emergence of bacterial resistance to antibiotics, and can certainly minimise side-effects.