Probiotics can be defined as live microbial food supplements which beneficially affect the health of the host by maintaining a balanced intestinal flora and stimulating its immune system. Although microbes have been used for many decades for food and alcoholic fermentation, they have only recently drawn the attention of the scientific community. Probiotic microorganisms are mostly from the Lactobacillus and Bifidobacterium family, and may contribute to our health in many different ways. Beneficial health effects attributed to probiotics are shortening of the duration of rotavirus diarrhea, relief of signs and symptoms of lactose intolerance, decreasing the risk of allergy in atopic individuals, cancer prevention, lowering of serum cholesterol levels, prevention of urogenital infections, and synthesis and enhancement of the bioavailability of nutrients. Probiotic bacteria may compete with pathogens for nutrients and mucosal adherence, produce antimicrobial substances, and modulate mucosal immune functions. The beneficial effects of probiotics are strain-specific, therefore there is need for the definition of which probiotics (as a single strain or a combination) are most effective in specific diseases. Prebiotics are indigestible oligosaccharides that promote the growth or activity of a limited number of beneficial bacteria in the gut. Some of the typical prebiotics include fructo-oligosaccharides, galacto-oligosaccharides, inulin, oligofructose, xylo-oligosaccharides, acidic oligosaccharides, and resistant starch. The breakdown of prebiotic molecules by bacterial enzymes in the large bowel yields lactate and short-chain fatty acids (acetate, butyrate, and propionate) as end-products. Short-chain fatty acids are crucial for gut integrity and function, modulation of the immune system, calcium and magnesium absorption, and maintenance of normal serum cholesterol levels. The end products of this fermentation are consumed by both these bacteria and intestinal epithelium as fuel. Synbiotics refer to nutritional supplements which contain probiotics and prebiotics in combination. Since it has been hypothesized that prebiotics ensure the survivability of some beneficial bacteria, their effects might be additive or even synergistic. Anahtar Kelimeler : probiyotikler, prebiyotikler, sinbiyotikler, bağırsak florası, sağlık,allerji, immünite, ishal, kanser, ürogenital sistem enfeksiyonları, serum kolesterol düzeyi, enflamatuvar bağırsak hastalıkları, probiotics, prebiotics, synbiotics, intestinal flora, health, allergy, immunity, diarrhea, cancer, urogenital infections, serum cholesterol, inflammatory bowel disease