Recently, there has been significant research interest both in academia and industry on the development of advanced polymer nanocomposite materials because of the improvement of performance for these class of materials, including their mechanical strength, toughness and stiffness, superior barrier properties to gases and moisture, and other functional properties depending on the type of reinforcements and polymer matrices. Cellulose nanoparticles are interesting nano-reinforcements for the fabrication of advanced polymer nanocomposite materials, due to their several interesting properties, namely low density, easy availability, low cost, and easy surface modification, and hence they can be incorporated into a wide range of polymer matrices. This chapter aims to review various cellulose nanoparticles as potential reinforcements for the fabrication of advanced polymer nanocomposites. First, an overview of cellulose nanoparticles is presented, followed by details of fabrication methods for cellulose nanoparticle-reinforced advanced polymer nanocomposites. After that, some of the properties of cellulose nanoparticle-reinforced polymer nanocomposites will be discussed. Finally, the chapter concludes with some of the potential applications of cellulose nanoparticle-reinforced advanced polymer nanocomposites.