Nowadays, various cellulosic biomaterials (absorbents, bandages, etc.) and substances (binders, thickeners, gelling agents, etc.) are being used in medicine, biology, and hygiene. Novel developments in these areas include nanoparticles, nanofibrils, and nanofilaments, made up of nanostructured cellulose, chitin, chitosan, and other cellulose derivatives. Biosynthesized nanofibrils can be used as implants, while artificial nanofibrils are thickeners and gellants of liquid systems. Cellulose fibers and nanofibers are used for the production of absorbing nonwoven bandages and active multilayer dressings for wounds, injuries, and burns. Carriers made up of modified cellulose containing therapeutically active substances can expand application areas of this natural polymer. The prime objective of this chapter is to discuss the production methods, structure, properties, and application areas of natural, artificial, and modified cellulosic materials, as well as micro-and nano-cellulosic materials and composites.