This chapter summarizes studies on the nanostructure-property interdependencies in tailored/structured polymer nanocomposites, and explains the nanoscale principles leading to the design and synthesis of high-performance nanomaterials for energy storage applications. It provides clues on the mitigation of such detrimental effects, which are commonly responsible for the poor dielectric performance of the composites. Three key factors are emphasized for synthesizing multifunctional nanocomposites: Nanoparticle functionality, Self-assembly of desired nano structures, and Hierarchical nanostructures. The chapter discusses fundamental interrelationships between the nanoscale dielectric properties and the macroscopic properties of the nanocomposites with an emphasis on the dielectric breakdown strength. It describes spectroscopic studies for composites with potential dielectrical applications and with promise to store electrostatic energy. Nanoparticle functionality and nanoparticles with controlled size have shown notable benefits against detrimental effects associated with the interfaces; however, certain applications may necessitate the integration of two or more inorganic phases.