Most intra-abdominal infections develop from a source in the gastrointestinal tract. They are usually caused by aerobic and anaerobic enteric bacteria. Management generally involves an invasive procedure to control the source of the infection and antimicrobial therapy directed against the causative microorganisms. In a few highly select patients, these infections may be treated without a definitive source control procedure. Antimicrobial therapy is tailored to the individual patient, with narrower spectrum agents used to treat community-acquired intraabdominal infections, and broader spectrum agents used for hospital-acquired infections. Overall, these infections remain associated with significant morbidity and mortality, particularly in higher-risk patients who have impaired host defenses.