Probiotics consist of bacteria that can be ingested and act by improving the microbiome and its function, enhancing digestion of food and by modulating the individual’s innate and immune response. To be effective, a probiotic microbe must be able to reach, proliferate, and colonize, even temporarily, the digestive tract. In addition to facilitating normal bowel function, probiotics have also been used as prophylactics and therapies for inflammatory bowel disease and Clostridium difficile–associated diarrhea. There are many different formulations of probiotics and they differ in activity and potency. Probiotic therapies seem to be a benign adjunct to better-understood treatments that are used for infectious and inflammatory diseases.