Traditionally the subject of nanomaterials and nanoscience belonged to physicists and chemists over the last several decades since the legendary lecture given by Nobel laureate Richard P Feynman (Physics) in 1959, "There's Plenty of Room at the Bottom". However, the idea of nanoparticle reinforced composites or nanocomposites came only about a decade ago after the remarkable discovery of carbon nanotubes and buckyballs. Infusion of these nanoparticles into various polymer matrices and their precursors opened up an important area of research in enhancing the properties of composite materials as we know today. The idea is synonymous with that expressed by Federal Agency such as National Nanotechnology Initiative (NNI) that "advances at the nanoscale will be meaningless if they cannot be interfaced well with the technology at larger material components, systems and architecture to produce usable devices". The development of structural nanocomposites from nanoparticle reinforced polymers addresses that concern precisely to the point. With this thematic idea of research in mind, there has been a flurry of activities within the last few years in US as well as in other countries, especially in the composite materials community to develop nanocomposites purely aiming structural applications. Phenomenal improvement in nanocomposite properties have been reported over a wide range of areas. The idea behind this monograph is to systematically describe the scientific basis of nanoparticle dispersion into various polymers, their interactions at the atomic scale, and their influence on the bulk properties of composite materials. The attempt will be to trace the source of such significant improvement in material properties and capture the momentum of this important area of research in materials science.