In the past decade, significant progress has been made in the area of advanced polymer materials, especially polymer nanocomposites. Future technology trends demand higher performance and lightweight materials in order to meet system level operational and reliability requirements. By combining the specific properties of the polymer matrix and the unique properties of nano-sized particles, a broad spectrum of properties of nanocomposites can be realized. Optimum combinations of mechanical, electrical and thermal properties of polymer materials can be tuned using proper choices of nanoparticles, coatings and processing conditions. In this chapter, recent developments on improving the mechanical and thermal properties of polymer nanocomposites will be summarized.