This review described the definition of pro-and prebiotics, and their functions and problems to be solved. Probiotics and prebiotics are functional Foods, which adjust the function and balance of intestinal microflora to be more beneficial and less risky to health. Probiotics in a viable form suppress the metabolism or population of harmful bacteria. Prebiotics are indigestible and fermentable oligosaccharides and polysaccharides including dietary fibers that are stimulators to growth of beneficial intestinal bacteria and consequently suppress the growth or metabolism of harmful bacteria. Probiotics sometimes supply with themselves the deficient or abnormal intestinal microflora that appear during and after the chemotherapeutic treatment, and correct an abnormal balance of the microflora in premature infants and elderly persons. The intestinal microflora may be hypothesized to consist of different bacteraial networks, which are not always in intact form. Probiotic bacteria supplement or activate the intestinal bacterial network with defects in respect of population level or metabolic activity. In this context intestinal bacteria other than bifidobacteria and lactic acid bacteria may be also candidates for probiotic bacteria. Prebiotics activate the bacterial metabolism and increase the population of the specific bacterial species to result in the satisfaction of the bacterial network beneficial to health. Pro-and prebiotics will be furthermore anticipated in future in maintenance or improvement of the human life without risk or with risk to health, respectively.