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Research in cellulose is exploding exponentially, as researchers look for renewable resources to replace chemicals and materials derived from fossil fuels. Cellulose is the most abundant biopolymer on earth and can be modified to make a large number of useful products, several of which will be discussed. Cellulose can be made into superabsorbent papers and gels, by introducing the appropriate amount of charge groups, usually carboxyl groups. Carboxylated fibers swell and can take up water more than hundred times their weight. They can be modified into an aqueous dope, from which one can spin textile fibers. The morphology of swollen fibers depends on the way the charges are introduced and reveals the underlying structure of the cell wall. By increasing the charge above 3 meq/g, the fibers break apart in nanocellulose and dissolved carboxylated cellulose (DCC). The nancellulose consist of cellulose nanofibers (CNF) and cellulose nanocrystals (CNC). The larger the charge, the larger the fraction of CNC. This CNC is different from conventional CNC, made by acid hydrolysis of cellulose fibers: it has DCC chains protruding from each end, which entangle when making films, resulting in transparent films much stronger than made from conventional CNC. They also have different cytotoxic properties: the higher the charge, the more they affect the metabolism of certain cells. The DCC end chains of CNC can by removed by hydrolysis, resulting in CNC with negatively charged ends. These particles can be crosslinked to make much longer chains, resembling CNF. Carboxyl groups in cellulose can be readily transformed in other functional groups, e.g. by a bioconjugation reaction. CNC films can be made superhydrophobic and paper can be made functional, making it hydrophobic, fluorescent or introducing sensors that can detect pathogens.