The choice of appropriate antimicrobial agents should take into consideration not only the interests of the individual patient, but also the ecological impact of different drugs and their delivery schedules. Selection of antibiotic-resistant organisms is a key aspect to remember. Bacterial populations harboring determinants of antibiotic resistance will be selected for by a range of antibiotic concentrations which are able to suppress or slow the growth of susceptible populations. These concentrations (selective concentrations) will be achieved within the human body in a series of compartments (selective compartments), where the potential selective power will be roughly proportional to the time of exposure of the bacteria to the drug (selective period). The duration of the expected exposure of bacterial populations to these concentrations of the drugs and the number of challenges they experience are probably the most important factors in predicting the potential selective activity of an antibiotic regimen. Such a risk analysis procedure may be used to propose guidelines for minimizing the development of antibiotic resistance.