Probiotics are traditionally defined as viable microorganisms that have a beneficial effect in the prevention and treatment of pathologic conditions when they are ingested. Although there is a relatively large volume of literature that supports the use of probiotics to prevent or treat intestinal disorders, the scientific basis behind probiotic use has only recently been established, and clinical studies on this topic are just beginning to get published. Currently, the best studied probiotics are lactic acid bacteria, particularly Lactobacillus and Bifidobacterium species. Other organisms used as probiotics in humans include Escherichia coli, Streptococcus sp., Enterococcus sp., Bacteroides sp., Bacillus sp., Propionibacterium sp., and various fungi, and some probiotic preparations contain more than one bacterial strain. Probiotic use for the prevention and treatment of antibiotic-associated diarrhea caused by Clostridium difficile induced intestinal disease as well as for other gastrointestinal disorders has been discussed in this review.