Engineering polymers, despite experiencing rapid increases in many applications including in space, nuclear reactors and in the medical field, suffer from limitations due to lack of sufficient strength and stiffness, electrical and thermal conductivities as well as thermal stability, etc. The advances in polymer nanocomposite technology, with the filler dimensions in nanometer scales, have made inroads into engineering polymers' applications in the clean energy, semiconductor, medical and construction industries owing to their significantly larger surface to volume ratio and the highly reduced particleparticle distance compared to their micro-sized particle counterpart. The quantum effects which begin to dominate the behaviour at the nano-scale, particularly at the lower end affect the fillers' optical, mechanical, electrical and magnetic behaviour. The scale at which discontinuity compared to bulk emerges at room temperature is different for different properties, e.g., for magnetic properties is typically ~50nm.The Bohr radius defining the optical and electronic properties is ~10nm.The critical nucleus size is 5 nm above which new crystalline phase will form, below which the clusters tend to dissolve rather than grow. When the polymeric materials are reinforced with smaller amounts of nanofillers, due to the evolution of extraordinary range of size dependent properties on these nanofillers, the filler-polymer matrix exhibit superior barrier properties, toughness, tribological and mechanical performance, optical properties, lower melting point, increased surface area, higher specific heat, increased electrical conductivity and magnetic properties showing decreased coercive field and increased remnant magnetization, which are not possible in conventional polymer composites. For example, bulk gold appears yellow in colour while nanosized gold appears red in colour. Large ZnO particles scatter visible light, and appear white, while nanosized ZnO particles, due to their smaller dimensions compared to the wavelength of visible light don't scatter it and appear clear. This is attributed to the shift in optical absorption toward shorter wavelengths as the size tends to be smaller. There is also deviation in melting point from the bulk value greater than a couple of hundred degrees when the particle size goes down to below 10 nm. Fracture strength of nanostructured Cu- Fe alloys, 2.8 GPa (at ~38 nm) is about 5 times the fracture stress of iron with larger grain sizes, ranging from 50 to 150 μm. Saturation magnetization, Ms, of zinc ferrite increases significantly below 20 nm from 1 to 4.5. The coercive field of Nd2Fe14B with nanograin structure decreases significantly and the remnant magnetization increases below ~40 nm. Silica micro-particles at 16 % volume show increased energy release rate by 40 %, while silica nano particles at 5 % and 10 % volume show increased energy release rate by 60 % and 86 % respectively, i.e., from 0.162 kJ/m2 to 0.26 and 0.3 kJ/m2. Electrical conductivity of neat HDPE is 1×10-9 Sm-1. However, with the addition of 10 wt % single-walled carbon nanotube (SWCNT), the electrical conductivity of HDPE increases up to 100 Sm-1. Similarly thermal conductivity of neat HDPE is only 0.52 Wm-1K-1. However, it increases by 573% with an addition of 0.2 vol% SWCNT reinforcement. The elongation at break becomes 2.3 times higher when the standard composites are reinforced with nanoparticles. Impact strength increases by 1.7 times as well as the bending strength and the modulus also increase due to the incorporation of the nanofillers. The emergence of the novel properties in polymer composites very much depend on the interfacial interaction between the polymer and these fillers. Thermodynamic and kinetic barriers inhibit the dispersal of inorganic nanoparticles with generally high surface energies within hydrophobic polymer matrices. In this chapter we discuss the effects of different types of interactions, including the intrinsic vs extrinsic properties of the nanofillers, as a result of their functionality, shape, orientation and size of the particles governing their properties.