Antimicrobials are often used in the surgical patient in an effort to prevent infection (prophylactic) or to treat established infection (therapeutic). To be effective, prophylactic antimicrobials at appropriate concentrations must be present in tissues at the surgical site at the time of contamination to prevent bacterial growth and subsequent infection. Therapeutic antimicrobials are used to treat established localized or systemic infection. Selection of antimicrobial agents for prophylactic or therapeutic use should be based on knowledge of expected flora, ability of the antimicrobial to reach the target tissue at appropriate concentrations, bacterial resistance patterns, drug pharmacokinetics, and culture and susceptibility testing results (therapeutic use). Failure of antimicrobial therapy to prevent or treat infection in the surgical patient may result from poor antimicrobial selection, inappropriate dosage or frequency, or inappropriate duration of therapy.