Since the early observations of Elie Metchnikoff, a wealth of experiments have described the use of selected microorganisms, mainly belonging to the lactic acid bacteria family, for the prevention or treatment of a variety of pathological situations. Nevertheless, the mechanisms underlying the proposed actions remain vastly unknown, partly as a consequence of the complexity of the gastro-intestinal ecosystem with which these biotherapeutic agents are expected to interact, but also because of the increasing variety of strains considered to have potential probiotic characteristics. During the past decades, however, the beneficial effect of specific strains in preventing or treating intestinal disorders has been substantiated by well-controlled clinical trials. Increasing evidence, including human studies, is also supporting the immunomodulatory role attributed to given lactic acid bacterial strains. The desire by consumers to use natural methods for health maintenance rather than long-term chemotherapeutic agents (i.e. antibiotics), linked to their expectation that food becomes a source of prolonged well-being, supports the speculation that the probiotic market will expand rapidly. Much of this growth will also depend on the reliability of claims that these products will bare. Therefore, the legislator will have to provide clear rules and regulations which will depend on measurable biomarkers and criteria based on scientific evidence. These commercial and legislative needs will hopefully provide scientists with the resources necessary to conduct the multidisciplinary research required to establish facts and mechanisms of action for carefully selected probiotic strains. These research results will probably be as essential for the positioning of probiotic preparations as either a food, a food supplement or as pharmaceutical preparation.