Biopolymer chitin, the most abundant natural amino polysaccharide, and its most important derivative, chitosan, are recently considered as the subjects for extensive worldwide academic and industrial research. In spite of potential applications of chitin and chitosan, it is necessary to establish efficient appropriate modifications to explore fully their high potential. A variety of chemical modifications are employed to modify these carbohydrate polymers. The present article provides a comprehensive review on one of the most promising approaches to modify chitin and chitosan, i.e., graft copolymerization, with an emphasis on the synthetic aspects. Both chemically- and radiation-initiated graft copolymerization of various vinyl monomers onto the trunk polymers are investigated. Meanwhile, the limited cases of polycondensation and oxidative coupling are presented as the non-vinyl graft copolymerization methods. Then, the ring-opening graft copolymerization is described and the cases of the cyclic monomers a-aminoacid N-carboxy anhydrides and e-caprolactone are investigated. An extensive description of the “grafting onto” approach is provided. The preformed polymers discussed here for grafting onto chitin/chitosan include living poly(2-alkyl oxazolines), poly(ethylene glycol)s, block polyethers, poly(ethylene imine)s, poly(2-hydroxyalkanoate)s, polyurethanes, poly (dimethylsiloxane)s, and dendrimer-like hyperbranched polymers. Chitin/chitosan multiple modification including graft copolymerization is also investigated. Regioselective grafting using derivatives such as 6-iodo-, mercapto-, deoxy(thiosulphato)-chitins, and Ntrichloroacetyl chitosan are described as suitable approaches to achieve chitin/chitosan graft copolymers with well-known structures.