This article is part 1 of a two-part review of inulin-type prebiotics. Prebiotics are a category of nutritional compounds grouped together by the ability to promote the growth of specific beneficial (probiotic) gut bacteria. Inulin-type prebiotics contain fructans of the inulin-type. Fructans are a category of nutritional compounds that encompasses naturally occurring plant oligo- and polysaccharides in which one or more fructosyl-fructose linkages comprise the majority of glycosidic bonds. To be inulin-type a fructan must have beta (2(1) fructosyl-fructose glycosidic bonds, which gives inulin its unique structural and physiological properties, allowing it to resist enzymatic hydrolysis by human salivary and small intestinal digestive enzymes. Inulin-type prebiotics include fructooligosaccharides (FOS), oligofructose, and inulin - terms that have been used inconsistently in both the scientific literature and in food applications. Commercially available inulin-type prebiotics can be extracted from food (typically chicory root) or synthesized from a more fundamental molecule (typically sucrose). Depending on the starting source and degree of processing, inulin-type prebiotics can be produced with very different chemical compositions. Some inulin-type prebiotics are relatively high in free sugars (the monosaccharides fructose and glucose and the disaccharide sucrose), while others have most or all free sugars removed. Processing can also result in mixes consisting exclusively of inulin-type oligosaccharides, polysaccharides, or both. Because inulin, oligofructose, and FOS resist enzymatic digestion in the upper gastrointestinal tract, they reach the colon virtually intact where they undergo bacterial fermentation. All inulin-type prebiotics are bifidogenic - stimulating the growth of Bifidobacteria species. The effects they have on other gut organisms are less consistent. A minimal dose of inulin-type prebiotic appears to be needed to produce a bifidogenic effect. However, intra-individual response to an identical dose of the same inulin-type prebiotic, in terms of stimulation of total number of Bifidobacteria and individual Bifidobacteria species, can be variable. Research on therapeutic uses of inulin-type prebiotics will be covered in part 2 of this review.