Cellulose is the most frequent polysaccharide in nature consisting of (some hundreds up to ten thousands) β-glycosidic linked glucose molecules. It is the main constituent of plant cell walls and vegetable fibre. It occurs mostly associated with hemicelluloses and lignin. It is therefore a common component of plant-based feed for all food producing and companion animals. However, these animals are not capable to digest cellulose enzymatically due to the lack of cellulases. The monomer element of cellulose, glucose, will not be released from cellulose. But gastrointestinal microbes can split cellulose, the main degradation products are short-chain fatty acids. In a simplified view, monogastric animals cannot digest cellulose, small amounts are microbially degraded in the large intestine. Minor amounts of cellulose may be absorbed as such by paracellular transport (passing through the intercellular space) or by transcytosis (transcellular transport of macromolecules captured in vesicles). On the other side, animals with large fermentation chambers in the intestine, such as ruminants, horses and rabbits, utilise large amounts of cellulose as energy source. In summary, cellulose is a natural part of feed and plays a physiological role in nutrition of animals (see Section 3.2.1).