Intra-abdominal infections frequently occur after trauma, surgical resections, or intrinsic diseases of the gastrointestinal tract. These infections, which can be generalized or localized in intraperitoneal or extraperitoneal locations as well as in organs, are often difficult to diagnose and treat. They are usually polymicrobial, involving both aerobic and anaerobic bacteria. The average number of bacterial strains recovered is between two and five, with anaerobes dominating. The aerobic microorganisms most commonly recovered are Enterobacteriaceae (mainly Escherichia coli) and enterococci (mainly Enterococcus faecalis). Among the anaerobic microorganisms, Bacteroides fragilis, anaerobic cocci, and clostridia are usually isolated. Adequate surgical drainage and antimicrobial therapy improve the clinical course of intra-abdominal infections and reduce the risk of local complications and septicemia. The choice of antimicrobial agents should be based on the type of infection, the microorganisms most likely to be encountered, and the microbial sensitivity pattern of the clinic. The side-effect profile of the antimicrobial therapy should also be taken into consideration.