Background: Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit for the host. This review summarizes current (i.e. published in the last 5 years) key evidence on probiotic efficacy and its safety in adults and children. To identify relevant data, searches of MEDLINE and the Cochrane Library databases were performed in August 2015 to locate randomized controlled trials (RCTs) or their meta-analyses. The focus was on commonly used, well-specified, bacterial and yeast probiotics for the treatment of gastrointestinal disorders. The MEDLINE database was also searched for evidence-based clinical practice guidelines, developed by scientific societies in the same timeframe. Data on safety of probiotics were obtained from a document developed by the US Agency for Healthcare Research and Quality. Key Messages: A number of relevant RCTs and meta-analyses are available. Saccharomyces boulardii is the most studied yeast probiotic, and Lactobacillus rhamnosus GG is the most studied bacterial probiotic. For both, the best documented fact is their efficacy for the treatment of acute gastroenteritis, especially in children, and for the prevention of antibiotic-associated diarrhea, both in adults and children. There is some evidence to support the use of probiotics to prevent or treat other diseases, such as necrotizing enterocolitis, infantile colic, Helicobacter pylori infection, and irritable bowel syndrome, but further studies are needed to identify which strain(s) is/are the most effective. Data on safety, particularly long-term safety, are limited. The risk of side effects is greater in people who have severe underlying health conditions. Conclusions: The evidence on bacterial and yeast probiotics has considerably expanded during recent years. Accumulated data allow one to make informed decisions about the effectiveness of probiotics and about how to reduce the use of those without proven efficacy.