Macro- and nanocellulose fibers have attracted research attention due to their remarkable properties like biodegradability, high strength, and stiffness, as well as their synthesis and application for development of composites. Macro- and nanocellulose fibers can be employed as a reinforcement agent in composite materials due to their outstanding biodegradation, thermal, and mechanical properties. Due to the hydrophilic nature of cellulose fibers, it is essential to enhance their surface irregularity for the production of composites with improved properties. In this chapter, we have discussed the surface alteration of cellulose nanofibers by various processes. Different synthesis techniques, followed by ionic liquids, properties, categories, and different applications of nanocellulose and their composites, have been examined in this chapter. © 2017 Elsevier Ltd. All rights reserved.