Polymer nanocomposites, consisting of a polymer matrix reinforced with nanofillers, significantly improve the properties of the material. This chapter explores the critical relationship between the original reinforcement’s characteristics and how surface functionalization affects the properties of the final or resultant nanocomposite. The intrinsic properties of different nanofillers, including carbon nanotubes (CNTs), graphene, and clay minerals, and their influence on the initial characteristics of the nanocomposite have been discussed in this chapter. It further covers the impact of several surface modification techniques, including chemical grafting, polymer coating, and electrostatic interactions, on interfacial adhesion, dispersion, and the resulting electrical, mechanical, thermal, and barrier properties of nanocomposites. To attain the optimum performance improvements, the chapter highlights the significance of a tailored strategy for surface functionalization that considers the unique characteristics of both the nanofiller and polymer matrix in a polymer nanocomposite system.