Polymer nanocomposites are materials that introduce nanoparticles into polymer matrices to change or improve the characteristics of the underlying polymer. This family of materials’ exceptional mechanical, thermal, electrical, and barrier qualities might open up new uses in the electronics, automotive, aerospace, and biomedical industries. Metal oxides, graphene, carbon nanotubes, and nanoclays are examples of high aspect ratio and surface area nanoparticles that form strong interfacial interactions with the polymer matrix and give rise to the unique characteristics of polymer nanocomposites. The types of nanoparticles employed, such as carbon-based nanomaterials, inorganic nanoparticle-modified polymers, and organic nanoparticles, have been described in this chapter. Additionally included are synthesis techniques such as melt blending, solution blending, and in situ polymerization. Additionally covered the characterization techniques such as mechanical testing, thermal analysis, and structural analysis. Important performance-influencing factors are also covered, including dispersion and distribution, interfacial adhesion, and nanoparticle orientation. Additionally, challenges and solutions are described. Finally, the use of polymeric nanocomposite is discussed as well.