The use of probiotics for the prevention and treatment of disease is still surrounded by controversy. Instead of providing a clear answer, the large body of data from randomized controlled trials has only served to heighten the debate. What are the reasons behind this controversy, and can we glean some valuable information from this extensive body of research? Professor Hania Szajewska tackles this daunting question using a simple approach. First, she performs high quality meta-analyses with data from at least 3-4 trials including a minimum of 1000 patients. Next, she separates the clinical effect of individual probiotic strains in order to identify the strains which can influence the clinical outcome.By applying these principles, Szajewska explores the potential of various probiotic strains in the prevention or treatment of acute gastroenteritis, antibiotic-associated diarrhea, nosocomial diarrhea, infantile colic, respiratory tract infections, and concludes with necrotizing enterocolitis in pre-term infants, a topic of hot debate. The benefits of probiotics are clear for some indications, but further work is needed in others. Szajewska ends by highlighting the importance of evaluating the effects of individual strains and using only those with proven effects in the clinic.