Lignocellulosic biomass is a renewable resource that consists mainly of three types of biopolymers: cellulose, hemicellulose, and lignin. It is a fossil alternative that is highly available in the form of agricultural residues, herbaceous crops, woody crops, forestry residues, waste paper, and other wastes from municipal and industrial uses. Cellulose is a major constituent with unique characteristics, especially depolymerization into its nanodimension (nanocellulose). These types of biomaterials are equipped with superior properties such as lightweight structure, high aspect ratio with durability, large surface area, tunable reactive surface, exceptional mechanical properties, high crystallinity, sustainability, 248and biodegradability. The excellent features have made both cellulose and nanocellulose have a wide range of applications in the construction, packaging, automobile, transportation, and biomedical fields. However, there are also challenges in mimicking this biomaterial as a conventional petrol-based material in order to well suit and be compatible with existing matrices. Thus, tailoring the surface functionality of cellulose and nanocellulose via different types of functionalization (e.g., chemical functionalization, organic compound functionalization, macromolecular functionalization, and inorganic compound functionalization) is an important route to improving the performance of pure biomaterials. This chapter highlights the types of biomass resources for cellulose and nanocellulose; chemical functionalization to different types of derivatives; physicochemical properties of cellulose, nanocellulose, and its derivatives; and application of the functionalized materials for nanocomposites.