[The mammalian intestinal tract contains a complex, dynamic and diverse society of microorganisms. The beneficial effects of developing a normal bacterial flora are: colonic resistance against pathogens, immunmodulation and intact intestinal barrier. Probiotics are live microbial supplements which beneficially affect the host by improving its intestinal microbial balance after oral administration. The health benefits of probiotics have been the subject of increased research interests. This paper gives a review of the literature that study the roles of probiotics in the prevention and treatment of antibiotic-associated diarrhea, traveller's diarrhea, irritable bowel syndrome, inflammatory bowel disease, Helicobacter pylori infection and hepatic encephalopathy. In human studies the examined probiotics are safe, tolerable and seem to be effective in conditions of diarrhea caused by antibiotics, traveller's diarrhea and pouchitis. In other above-mentioned conditions further randomized and controlled clinical trials are needed to evaluate their efficacy. Based on these results, in the research and manufacturing of genetically-engineered probiotic bacteria a major leap is expected.]