For decades, if not centuries, a variety of products with what would now be regarded as prebiotic and probiotic properties have been consumed by the general public and advocated for their benefits on health and, in particular, gastrointestinal well-being. More recently, medical science has taken a great interest in the population of micro-organisms, the gut microbiota that normally populates the human gut, and the range of important functions carried out by the microbiota in health is being progressively defined. As a corollary, the list of disorders and diseases that may result from disruption of the normal microbiota and/or its interaction with the host continues to grow. A scientific basis for the use of probiotics and prebiotics is, therefore, beginning to emerge. Unfortunately, although progress has been made, the clinical evidence to support the use of these preparations lags behind. Nevertheless, a number of human disease states may benefit from the use of probiotics, most notably, diarrheal illnesses, some inflammatory bowel diseases, certain infectious disorders, and irritable bowel syndrome. Prebiotics promote the growth of "good" bacteria, and although a variety of health benefits have been attributed to their use, prebiotics have been subjected to few large-scale clinical trials.