This chapter is aimed to thoroughly describe the thermal degradation of cellulose and cellulosic substrates (like cotton and paper), investigating the involved reaction mechanisms. After a brief introduction about the real reasons that motivated the study on cellulose degradation, a first general introduction on cellulose thermal degradation (namely, pyrolysis) and thermooxidation will be presented, followed by a detailed description of the related mechanisms with particular attention to the main degradation products consisting of volatile species and char (carbonaceous residue). To this aim, some of the most important factors affecting the volatilisation and charring balance (i.e. heating rate, presence of humidity, metals and flame retardants) are considered and thoroughly discussed in order to provide a complete scenario about the cellulose degradation phenomenon.