The gut contains a diverse bacterial flora that is acquired at birth and has a number of physiological functions. Administration of prebiotics or probiotics may favourably alter this gut microflora. Prebiotics are poorly digested oligosaccharides that promote the growth of desirable bacteria and may have other beneficial gastrointestinal and systemic effects. Probiotics are "helpful" human bacteria that provide a variety of health benefits when administered exogenously. Probiotics produce beneficial effects in the prevention and treatment of traveller's diarrhoea, viral diarrhoea, and diarrhoea in day care centres. Moreover, probiotics have been shown to reduce relapses associated with Clostridium difficile, and Lactobacilli are effective in the prevention of antibiotic-associated diarrhoea. Probiotics may also be efficacious in the treatment of gastroenteritis. Clinical studies of probiotics in inflammatory bowel disease have proved disappointing, but beneficial effects in adults with irritable bowel syndrome have been reported with Bifidobacterium infantis 35624. Lactobacilli GG reduces the incidence of gastrointestinal symptoms and gut permeability in patients with atopic dermatitis, and administration of probiotics reduces the frequency and severity of atopic eczema when administered to pregnant women and then to newborn infants. In conclusion, probiotics are effective in the treatment and/or prevention of a number of conditions, including diarrhoea, irritable bowel syndrome and atopic dermatitis, and the product used should be selected based on the particular indication.