Probiotics are increasingly used for prevention and treatment of diarrhea more in children than in adults. Given the broad spectrum of diarrhea, this review focuses on the main etiologies: acute gastroenteritis, antibiotic-associated diarrhea (AAD), and necrotizing enterocolitis (NEC). For each, we reviewed randomized controlled trials, meta-analyses, and guidelines. For acute gastroenteritis we found 12 guidelines: 5 recommended probiotics and 7 did not. However, the guidelines containing positive recommendations provided proof of evidence from clinical trials and meta-analyses. Lactobacillus rhamnosus GG (LGG) and Saccharomyces boulardii had the most compelling evidence of efficacy as they reduced the duration of the disease by 1 day. For AAD 4 meta-analyses were found, reporting variable efficacy of probiotics in preventing diarrhea, based on the setting, patient's age, and antibiotics. The most effective strains were LGG and S. boulardii. For NEC, we found 3 randomized controlled trials, 5 meta-analyses, and 4 position papers. Probiotics reduced the risk of NEC enterocolitis and mortality in preterm babies. Guidelines did not support a routine use of probiotics and asked for further data for such sensitive implications. In conclusion, there is strong and solid proof of efficacy of probiotics as active treatment of gastroenteritis in addition to rehydration. There is solid evidence that probiotics have some efficacy in prevention of AAD, but the number needed to treat is an issue. For both etiologies LGG and S. boulardii have the strongest evidence. In NEC the indications are more debated, yet on the basis of available data and their implications, probiotics should be carefully considered.