Chitosan, a natural based polymer obtained by alkaline deacetylation of chitin, is non-toxic, biocompatible, and biodegradable. These properties make chitosan a good candidate for the development of conventional and novel drug delivery systems. Recently, there has been a growing interest in the chemical modification of chitosan in order to improve its solubility and widen its applications. Chemical modification of chitosan is useful for the association of bioactive molecules to the polymer and controlling the drug release profile. Chemical modification will introduce desired properties and enlarge the field of the potential applications of chitosan with the choice of various types of side chains.