Nanofibrillar celluloses are promising new bio-based nanomaterials that can be prepared from paper- grade chemical pulps and other plant celluloses by mechanical shearing in water, usually after pretreatments. For example, enzymatic hydrolysis, carboxymethylation, addition of cationic polymers, TEMPO-mediated oxidation and others have been applied as wood cellulose pretreatments to reduce the energy consumption of the mechanical shearing process and to improve nanofibrillation level. Nanofibrillated celluloses (NFCs) prepared from wood cellulose by either enzymatic hydrolysis or partial carboxymethylation and subsequent mechanical shearing in water are convertible to nanopaper films and aerogels using a filtration process like that used in papermaking, which is advantageous for efficient removal of water from the strongly swollen NFC/water dispersions. NFCs have high molecular weights and long fibrils and form fibril network structures both in aqueous dispersions and dried nanopaper films/aerogels. This makes them preferable for use as base materials for nanocomposites. Thus, various nanopaper/matrix composites have been prepared, some of which show remarkably high mechanical strength including high ductility. When TEMPO- mediated oxidation is used as the pretreatment, almost completely individualized TEMPO-o xidized cellulose nanofibrils (TOCNs) with homogeneous widths of ~3 nm dispersed in water can be prepared from oxidized wood celluloses with carboxylate contents >1.2 mmol/g by gentle mechanical disintegration treatment. Because TOCN elements form nematic-ordered structures due to their self- assembling behavior in water, TOCNs are able to be converted to dense films with plywood- like layered structures, stiff hydrogels by acid treatment, aerogels with extremely high specific surface areas, and other unique bulk materials. When TOCNs are used to make nanocomposite materials, high mechanical strengths and gas- barrier properties can be achieved even with low TOCN-loading ratios.