Peritonitis is a serious and common infection. Its pathogenesis and microbiology have been well defined. Such risk factors as age, site of infection, physiologic response of the patient, presence of organ dysfunction, and malnutrition may influence the outcome of this disease process. The presence of antibiotic-resistant organisms and delays in operative intervention are also associated with treatment failure and higher mortality. Surgeons have the greatest impact on this disease in their ability to control the source of infection and to administer proper antimicrobial therapy.