Arabinoxylans (AXs) are non-starch polysaccharides with complex structures naturally occurring in grains (i.e., barley, corn, and others) providing many health benefits, especially as prebiotics. AXs can be classified as water-extractable (WEAX) and water-unextractable (WUAX) based on their solubility, with properties influenced by grain sources and extraction methods. A large body of evidence shows that AXs exert an im-portant health impact, including glucose and lipid metabolism regulation and immune system enhancement, which is induced by the interactions between AXs and the gut microbiota. Recent research underscores the dependence of AX physiological effects on structure, advocating for a deeper understanding of struc-ture-activity relationships. While systematic studies on WEAX are prevalent, knowledge gaps persist re-garding WUAX, despite its higher grain abundance. Thus, this review reports recent data on WUAX struc-tural properties (chemical structure, branching, MW) in cereals under different treatments. It discusses WUAX applications in baking and its benefits deriving from gut fermentation.