The human gastrointestinal tract is home for diverse and vast communities of micro-organisms representing about 500 cultivable species. Colonisation of the gastrointestinal tract begins immediately after birth. The mode of delivery, use of antibiotics, and the level of hygiene are known to exert a significant influence on the number and species of micro-organisms that colonise the gut. Transition to the adult flora occurs after weaning, and by the second year of life the intestinal flora becomes similar to that of an adult and remains relatively stable throughout life. Research over the past decades has provided evidence that administration of probiotics could be used to optimise the gut flora and to prevent and treat a range of diseases. Health benefits supported by adequate clinical data or promising animal data include prevention and treatment of diarrhoeal diseases (acute infantile diarrhoea, antibiotic associated diarrhoea). They are effective in prevention of systemic infections, inflammatory bowel disease, allergies, malignancies, hypercholesteronaemia, and in alleviation of lactose intolerance.