Interest in probiotics is at an all-time high in the United States, driven in part by new products emerging in the market, by US researchers eager to evaluate efficacy claims rigorously, and by consumers interested in potential therapeutic and preventive health benefits. The US marketplace is a mixed bag of products, some well-defined and properly evaluated in controlled clinical studies and others with unsubstantiated claims of efficacy. Validation of probiotic contents in commercial products is needed to ensure consumer confidence. The term "probiotic" should be used only for products that meet the scientific criteria for this term-namely, products that contain an adequate dose of live microbes that have been documented in target-host studies to confer a health benefit. Probiotics must be identified to the level of strain, must be characterized for the specific health target, and must be formulated into products using strains and doses shown to be efficacious. Several characteristics commonly presumed to be essential to probiotics, such as human origin and the ability to improve the balance of the intestinal microbiota, are discussed.