Probiotics have been extensively studied over the past several years in the prevention and, to a larger extent, in the treatment of diarrheal diseases, especially in pediatric populations. Diarrhea is a symptom, and not a disease. This review will not address chronic disorders associated with diarrhea, or Clostridium difficile-induced diarrhea. Rather it will focus on published clinical trials performed on acute-onset, likely infectious diarrhea occurring in the settings of day-care centers, in the community, acquired in the hospital, antibiotic-associated diarrhea, and treatment of acute infectious diarrhea. For prevention of diarrhea acquired in day-care centers, 9 randomized and placebo-controlled trials have been published, conducted in different parts of the world. Probiotics tested were Lactobacillus GG, Bifidobacterium lactis (alone or in combination with Streptococcus thermophilus, and Lactobacillus reuteri, Lactobacillus rhamnosus (not GG), and Lactobacillus acidophilus, in various trials either alone or in comparison with each other. The evidence of their efficacy in these settings is only modest for the prevention of diarrhea, although somewhat better for prevention of upper respiratory infections. In the community, new trails conducted in underprivileged areas of India, again with modest efficacy. Previous trials that examined the potential role of probiotics in preventing the spreading of diarrhea in hospitalized children had yielded conflicting results. More recently, a large trial in Poland showed, however, rather good evidence of efficacy for Lactobacillus GG. The prevention of antibiotic-associated diarrhea has been the subject of many investigations, both in children and in adults. Most commonly used probiotics were Lactobacillus GG, Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium ssp, Streptococcus ssp, and the yeast Saccharomyces boulardii. In general, most of these trials do show clear evidence of efficacy, with the 2 most effective strains being Lactobacillus GG and S. boulardii. Evidence is also emerging on the importance of the dose in reducing the incidence of this type of diarrhea, and the incidence of Clostridium difficile-associated postantibiotic diarrhea. As for treatment, a large body of data is available, especially in children, on the effect of several strains of probiotics in treating sporadic infectious diarrhea. The vast majority of the published trials show a statistically significant benefit and moderate clinical benefit of a few, well-identified probiotic strains—mostly Lactobacillus GG and S. boulardii—in the treatment of acute watery diarrhea, and particularly those due to rotavirus. Such a beneficial effect results, on average, in a reduction of diarrhea duration of approximately 1 day. The effect is strain-dependent and dose-dependent.