The gut flora plays a fundamental role in maintaining normal intestinal function. A disturbance of this flora, or the host response to this flora, has been clearly demonstrated to play a role in the pathogenesis of inflammatory bowel disease (IBD). This has led to attempts to modify the bacterial flora with "live non-pathogenic organisms that confer health benefits by improving the microbial balance," otherwise known as probiotics. Recent attention has focused on this potential strategy to treat or prevent IBD. The potential therapeutic benefit is enhanced by the natural and apparently safe approach that probiotics offer. Animal models of colitis have provided the proof of principle that probiotics can prevent and treat established intestinal inflammation. Controlled clinical studies have demonstrated the efficacy of probiotics in the maintenance of remission of pouchitis, prophylaxis of pouchitis after the formation of an ileoanal reservoir, maintenance of remission of ulcerative colitis, and treatment of Crohn's disease. However, large controlled trials are needed to definitively establish the place for probiotics in the treatment of IBD and resolve issues such as the dose, duration, frequency of treatment, and use of single or multiple strains. Research is focusing on establishing the mechanism of action, so that treatments with individually tailored properties are developed and innovative approaches are explored.