In the last several decades, solid polymer electrolytes (SPEs) for Li-ion battery applications have been intensively explored as a safer alternative to the conventional liquid electrolytes currently employed in commercial devices. Although SPEs offer attractive properties such as dimensional stability, flexibility, better interfacial contact with electrodes, and safety, to name a few, they cannot match the room temperature ionic conductivity of conventional liquid electrolytes. Various strategies, including polymer blending, copolymerization, use of plasticizers, and dispersion of nanofillers, have been explored to improve ionic conductivity. Incorporating inorganic nanofillers into the polymer matrix is thought to be the most promising strategy for achieving optimized battery performance. There are two main types of nanofillers used in composite solid polymer electrolytes (CSPEs): inert fillers and active fillers. Both types of additives enhance ionic conductivity via different mechanisms. As an example, the inert nanofiller generally enhances the conductivity by suppressing the crystallization of the host polymer (it is the amorphous domain of the polymer that allows ionic mobility). On the contrary, the active fillers are ionic conductors; hence, they can conduct through their bulk and facilitate Li-ions conduction through the matrix. This chapter focuses on the nanofillers employed in CSPEs. A description of the filler types, their role in enhancing the ion conductivity, ion-conducting mechanism, and attributes of the nanofiller that contribute to conducting efficiency of the CSPEs is given. Finally, the impact of nanofiller modification on the electrical performance of the CSPEs has also been discussed.