Chitin and chitosan are copolymers of N-acetyl-D-glucosamine (GlcNAc) and D-glucosamine (GlcN) units linked with β-(1-4) glycosidic bond, where the predominant units are GlcNAc for chitin or GlcN for chitosan in their polymeric chains (Fig. 1). While chitin remained an unused natural resource for a long term, interest in chitosan and chitooligosaccharides (COS) has increased in recent years due to their unique biodegradability, biorenewability, biocompatibility, physiological inertness, and hydrophilicity. Based on these properties, chitosan and COS have been widely and continuously applied in various fields, such as agriculture, cosmetics, water treatment, food industry, pharmaceuticals and biomedicine. Actually, most biological activities of chitosan are strongly dependent on its degree of polymerisation (DP) which defines the molecular mass of the polymers, degree of acetylation (DA) which defines its charge density and pattern of acetylation (PA) which defines the distribution of GlcNAc and GlcN moieties in the chitosan chain.