Episodes of acute infectious diarrhoea remain a major disease burden throughout the world, especially in developing countries. They are due to infection by many different organisms. Most episodes are self-limiting and usually investigations are not done to identify the infectious agent. The main risk to health is dehydration and management aims to improve and maintain hydration status. However, rehydration fluids do not reduce the stool volume or shorten the episode of diarrhoea. Probiotics are “friendly” bacteria that improve health and are not harmful in themselves. A number of randomized controlled trials have been done to see whether probiotics are beneficial in acute infectious diarrhoea. We have searched for as many of these trials as possible and collected together the data in a systematic way to try to discover whether or not probiotics are beneficial in acute diarrhoea. We identified 63 trials, which included a total of 8014 people – mainly infants and children. Probiotics were not associated with any adverse effects. Nearly all studies reported a shortened duration of diarrhoea and reduced stool frequency in people who received probiotics compared to the controls. Overall, probiotics reduced the duration of diarrhoea by around 25 hours, the risk of diarrhoea lasting four or more days by 59% and resulted in about one fewer diarrhoeal stool on day 2 after the intervention. However, there was very marked variability in the study findings and so these estimates are approximate. We concluded that these results were very encouraging but more research is needed to identify exactly which probiotics should be used for which groups of people, and also to assess the cost effectiveness of this treatment.