Treatment with antimicrobial agents must be tailored to the individual patient, the site of infection, and the etiologic organism involved. Effectiveness, toxicity, and cost are the basic considerations in choosing a drug. Because initiation of therapy often cannot be delayed until microbiologic studies have been performed, empiric treatment should be sufficiently broad to cover the most likely pathogens, based on the site of infection and the type of host. Definitive therapy may differ from initial therapy and should be instituted as soon as specific laboratory and clinical data are available. Information about allergic reactions to drugs should be elicited; because of the numerous families of antimicrobial agents currently available, most infections in patients with drug allergies can be treated with adequate substitutes. Cautious conservatism is advocated in the use of new antimicrobial agents. The effects of new agents on the microbial ecology and the hospital environment should be considered.