Appropriately administered antibiotic prophylaxis reduces the incidence of surgical wound infection. Prophylaxis is uniformly recommended for all clean-contaminated, contaminated and dirty procedures. It is considered optional for most clean procedures, although it may be indicated for certain patients and clean procedures that fulfil specific risk criteria. Timing of antibiotic administration is critical to efficacy. The first dose should always be given before the procedure, preferably within 15–60 minutes before incision. Re-administration at one to two half-lives of the antibiotic is recommended for the duration of the procedure. In general, postoperative administration is not recommended. The duration of administration is extended only in special circumstances, such as gross contamination secondary to a ruptured viscus or severe trauma. Antibiotic selection is influenced by the organism most commonly causing wound infection in the specific procedure and by the relative costs of available agents.