Foods containing probiotic bacteria fall within the functional foods category, since they provide health benefits over and above basic nutrition. Probiotics, prebiotics and synbiotics aimed at improving intestinal health currently represent the largest segment of the functional foods market in Europe, Japan and Australia. To address whether these are reality or myth, one needs to critically examine the scientific data that have resulted from an extensive array of clinical studies. In this respect, there is mounting evidence that ingestion of certain live bacterial cultures can exert particular health benefits, mainly in the gastrointestinal, but also in respiratory and urogenital tracts. The scientific evidence falls into three categories: those that are accepted as proven; those for which there are promising data, indicating a beneficial effect; and where there is either contradiction or lack of evidence, but which have nonetheless been promoted as probiotic traits. It is also important to emphasise that these studies have been done on relatively few strains, which in many cases have not been selected for the particular probiotic property for which they are now being examined. In the future, there may be a need to critically select probiotics targeted against certain disease states for efficacy in humans. Furthermore, for probiotic benefits to be realised by the consumer, the technological issues relating to the development of a product containing these bacteria in sufficient numbers in a stable live form that can withstand the product shelf-life need to be overcome. The ability to overcome these technological issues may be considered as important as proof of clinical efficacy in some cases. Furthermore, the probiotic food product must be regularly consumed in sufficient quantities to deliver the required 'dose' of live bacteria to the gut, keeping in mind the losses in viability typically encountered during gastric transit, and which can be influenced by the probiotic 'delivery vehicle'.