The ingestion of probiotics is associated with various beneficial effects on human health and modifies the physiological homeostasis of the intestinal flora. Probiotics are microorganisms with some particular characteristics: human origin, safety in human use, bile and acid resistance, survival in the intestine, at least temporary colonization of the human gut, adhesion to the mucosa and bacteriocine production. Thanks to these characteristics, probiotics block the invasion of human intestinal cells by the enteroinvasive bacteria. Furthermore, they should be able to stimulate and modulate the intestinal immune response, and to protect and stabilize the mucosal barrier. Finally, the efficacy of probiotics should be evident and documented with valid studies. All their properties should be maintained during processing and storage. Probiotics are usually used to protect the host from pathogens. With regard to this, they are useful in the prevention of antibiotic and traveler's diarrhea and they may play a role in the management of gastric Helicobacter pylori infection. Furthermore, their efficacy in the treatment of infectious diarrhea, in inflammatory bowel diseases, in pouchitis and in food allergy has been shown. Probiotics can improve the symptoms of irritable bowel syndrome and of lactose malabsorption. Finally, it has been suggested that such microorganisms may play a role in the prevention of carcinogenesis and of tumor growth.