Nanocelluloses have unique morphologies, characteristics, and surface nanostructures, and are prepared from abundant and renewable plant biomass resources. Therefore, expansion of the use of CO2‐accumulating nanocelluloses is expected to partly contribute to the establishment of a sustainable society and help overcome current global environmental issues. Nanocelluloses can be categorized into cellulose nanonetworks, cellulose nanofibrils, and cellulose nanocrystals, depending on their morphologies. All of these materials are first obtained as aqueous dispersions. In particular, cellulose nanofibrils have homogeneous ≈3 nm widths and average lengths of >500 nm, and significant amounts of charged groups are present on their surfaces. Such charged groups are formed by carboxymethylation, C6‐carboxylation, phosphorylation, phosphite esterification, xanthation, sulfate esterification, and C2/C3 dicarboxylation during the pretreatment of plant cellulose fibers before their conversion into cellulose nanofibrils via mechanical disintegration in water. These surface‐charged groups in nanocelluloses can be stoichiometrically counterion‐exchanged into diverse metal and alkylammonium ions, resulting in surface‐modified nanocelluloses with various new functions including hydrophobic, water‐resistant, catalytic, superdeodorant, and gas‐separation properties. However, many fundamental and application‐related issues facing nanocelluloses must first be overcome to enable their further expansion. Nanocelluloses have unique morphologies, characteristics, and surface nanostructures, and are prepared from abundant and renewable plant biomass resources. Nanocelluloses can be categorized into cellulose nanonetworks, cellulose nanofibrils, and cellulose nanocrystals, depending on their morphologies. Cellulose nanofibrils have significant amounts of charged groups on their surfaces. These surface‐charged groups can be stoichiometrically counterion‐exchanged to diverse metal and alkylammonium ions.