Cellulose and hemicellulose constitute the primary insoluble dietary fibers in plant cell walls. Cellulose forms linear chains of β-1,4-linked D-glucose units providing rigidity, while hemicellulose comprises branched heteropolymers of xylose, mannose, and glucose that cross-link with cellulose. In foods, they promote gastrointestinal health by increasing fecal bulk, aiding transit time, and supporting gut microbiota fermentation. These polymers resist human digestive enzymes, contributing to satiety and blood glucose regulation. Naturally abundant in grains, vegetables, and fruits, they are non-digestible, thermally stable up to decomposition, and classified as GRAS by regulatory bodies for food applications.