Probiotics consist of yeast or bacteria, especially lactic acid bacteria. They are available as capsules, powder, fermented milks or yoghurts. Probiotics exhibit strain-specific differences in their resistance to acid and bile, ability to colonise the gastrointestinal tract, clinical efficacy, and benefits to the health of the host. There is level I evidence for the use of probiotics in treating acute infectious diarrhoea and preventing antibiotic-associated diarrhoea, with Lactobacillus rhamnosus GG and Saccharomyces boulardii having the most evidence to support their use for these conditions. There is level II evidence that S. boulardii combined with high-dose vancomycin is more effective than the antibiotic alone in preventing recurrent Clostridium difficile diarrhoea. There is level I evidence that probiotics prevent traveller's diarrhoea. There is level I evidence for use of the high-potency probiotic VSL#3 in preventing pouchitis, and level II evidence for this agent in preventing relapse in patients with ulcerative colitis. Probiotics are generally regarded as safe and well tolerated. Some probiotics may be contraindicated in patients who are immunocompromised or have severe underlying illness, as they have been reported to cause fungaemia and bacteraemia.