Western medicine has only recently discovered that the intestinal microbiota is a major determinant of the well-being of the host. Although it would be oversimplifying to limit the benefits of breastfeeding compared to cow milk based infant formula to differences in gastrointestinal flora, the impact of the latter has been demonstrated beyond doubt. As a consequence, gastro intestinal flora manipulation with pre- and probiotics added to infant formula or food (mainly milk based products) and/or with food supplements have become a priority area of high quality research. The composition of intestinal microbiota can be manipulated with "biotics": antibiotics, prebiotics and probiotics. Commercialised pre- and probiotic products differ in composition and dose. Major threats to the concept of developing a major role for intestinal microbiota manipulation on health are the commercialisation of products claiming health benefits that have not been validated. Legislation of food supplements and medication differs substantially and allows commercialisation of poor quality food supplements, what will result in negative experiences. Medicinal products can only be advertised for which there is scientific proof of benefit that has been demonstrated with "the same product with the same dose in the same indication". Specificity of prebiotics and probiotics strains and product specificity are of importance, although high quality evidence for this assertion is missing. Dose-efficacy studies are urgently needed. Probiotics are "generally regarded as safe", but side effects such as septicemia and fungemia have sometimes been reported in high-risk situations.