Functional oligosaccharides (FOs) are indispensable components in food, feed, pharmaceuticals, cosmetics, and nutraceuticals preparations. They are polysaccharides containing the degree of polymerization of 2-10, not digested (or only partially digested) by gut enzymes, having no or less caloric values, stable at acidic pH, thermostable (>50 °C), and intact in bile salts. Consequently, FOs are neither digested nor degraded in the upper alimentary canal and reach the large intestine of the host. FOs confer several health benefits, such as proliferation of probiotic microbes, elimination of pathogen colonization, enhancement of mineral absorption, evacuation of heavy metal ions by increasing bowel movements, and selective proliferation of specific probiotic microbes in the gut. FOs can be categorized as sucrose based (fructo-oligosaccharides), lactose based (galacto-oligosaccharides and lacto-sucrose), starch related (isomalto/malto-oligosaccharides, trehalose, and cyclodextrins), soy-based oligosaccharides, and non-starch based (xylo/arabino-oligosaccharides, and pectin- and chitosan-based oligosaccharides). FOs are produced by chemical, enzymatic, and chemoenzymatic techniques, but the enzymatic method is preferred because it produces specific FOs under mild, environmentally friendly conditions. FOs are purified and characterized using membrane filtration, chromatography, and spectroscopy. The prebiotic function of FOs in the host's health has been studied using animal models and a simulated human intestinal microbial ecosystem. © 2019 American Chemical Society.