Intra-abdominal infections and their complications most often result from perforation of the hollow intra-abdominal viscera due to vascular compromise, obstruction, inflammation, and trauma. The five most common aetiologies are appendicitis, diverticulitis, biliary tract disease, pancreatitis and trauma. Development of an intra-abdominal abscess, the most serious complication, has been shown to be a biphasic process involving a synergistic relationship between aerobic and anaerobic bacteria found in the gastro-intestinal tract. Successful treatment results from surgical correction and drainage plus an antibiotic regimen effective against both groups of enteric organisms.