Bacterial and viral diarrhea remain important causes of morbidity and mortality throughout the world. Current concerns with medical costs, increasing frequency of antibiotic resistance, and overuse of antibiotics in general have redirected therapeutic approaches for diarrhea from traditional drugs to the use of living therapeutic organisms. Awareness of the important role of normal flora in the microecology of the intestines in fighting infection, along with the recent availability of well-controlled clinical trials for these agents, has brought microecologic therapies to the forefront of clinical practice.