Chitin is a polymer of of N-acetyl--D-glucosamine. The linkage between individual N-acetyl--D-glucosamine units is -1,4, giving it a structure similar to that of cellulose, except that the hydroxyl on carbon 2 of each residue is replaced by an acetylated amino group.

Chitin is widely distributed among the kingdoms of organisms. It is a minor constituent in most fungi and some algae, where it often substitutes for cellulose or other glucans. In dividing yeast cells, chitin is found in the septum that forms between the separating cells. The best known role of chitin, however, is in invertebrate animals; it constitutes a major structural material in the exoskeletons of many arthropods and mollusks. In many of these exoskeletons, chitin forms a matrix on which mineralization takes place, much as collagen acts as a matrix for mineral deposition in vertebrate bones. The evolutionary implications are interesting. As animals evolved to the size that made rigid body parts essential, quite different paths were taken. The vertebrates developed a mineral skeleton on a collagen matrix. Annelids such as earthworms also use collagen, but in a segmented exoskeleton. The arthropods and mollusks also developed exoskeletons, but theirs were built on chitin-a carbohydrate rather than a protein matrix.