Polymeric nanocomposites made by incorporating small amount of nanoscale inclusions into polymer matrices exhibit dramatic changes in thermomechanical properties over the pure polymers. The properties of the nanoscale fillers can be extraordinary, yet the significant changes observed cannot be due to the nanofillers alone. Enhancing their effect is the extremely significant role that the interphase plays in these systems. Given the enormous surface to volume ratio for nanoparticles, the interphase volume fraction can dwarf that of the inclusions themselves and percolate through the composite. In this talk, experimental evidence of the existence of this interphase region is presented for several nanofiller types via local and global glass transition changes and microscopy. We show that by properly controlled functionalization of the nanoscale inclusions, we can impact the properties of the interphase region and consequently control the properties of the nanocomposites. In conjunction with the experimental results, the viscoelastic behavior of multi-phase polymeric nanocomposites is modeled using a novel hybrid numerical-analytical approach that can effectively take into account the existence of the interphase region and be used to elucidate experimental results and aid in materials design. To investigate the concept of percolated interphase, a finite element approach is developed to study the impact of interphase zones on the overall properties of composite. The results have impact on potential commercial applications for nanocomposites including transparent conducting films, wear resistant coatings and hybrid systems for multifunctional performance including sensing and damage tolerance.