Chitin is a complex polysaccharide, a polymer that forms long chains which unite into fibers.

It is secreted in the ectoderm and commonly forms the exoskeleton of invertebrates. It is a material much like collagen, which forms the connective tissue of many advanced animals, but collagen is secreted in the mesoderm.

In the exoskeletons of invertebrates chitin is one component of a protein - chitin complex. Chitin may be decomposed by bacteria and digested by enzymes but, if buried, may persist for hundreds of millions of years and is known from rocks as old as Cambrian in age.