The large availability of cellulose and its high density of hydroxy groups justify the popularity of its derivatives in the techno-scientific literature and, in a few cases, their commercial success. In fact, the commercialization of key cellulose esters, such as cellulose nitrate, cellulose xanthate, and cellulose acetate (all discovered in the nineteenth century), preceded that of today’s commodity plastics. Esterification, which remains one of the most common chemical modifications for cellulose, implies the reaction of hydroxy groups with organic acid anhydrides, strong mineral acids, or carbon disulfide. The production of cellulose ethers, mostly developed during the first half of the twentieth century, also involves electrophilic reagents, generally organohalides and epoxides. For instance, the reaction of cellulose with chloroacetic acid produces carboxymethylcellulose, while ethylene oxide leads to hydroxyethylcellulose. Another common reaction on hydroxy groups, oxidation, results in niche materials such as 6-carboxycellylose and 2,3-dialdehyde cellulose. Although oxidized cellulose is scarcely marketed as such, this modification is one of the most prominent pre-treatments leading to nanocellulose. In this sense, researchers recently tend to focus on modified nanofibers or nanocrystals. Moreover, there is growing interest in unconventional processes such as phosphorylation, silane coupling, amidation, urethanization, enzymatic reactions, grafting, and click chemistry.