Polymer diffusion and matrix morphology are investigated in polymer nanocomposites (PNCs). Using elastic recoil detection, tracer diffusion coefficients (D) of dPS and dPMMA are measured in PNCs containing polystyrene (PS) with phenyl-capped (Ph) silica, and poly(methyl methacrylate) (PMMA) with hydroxyl-terminated (OH) silica nanoparticles (NPs), respectively. As NP volume fraction increases or NP size decreases, polymer diffusion slows down. Moreover, D plotted relative to diffusion in a pure PS matrix (i.e., D/D0) falls on a master curve when plotted against the interparticle distance of NPs relative to the tracer radius of gyration, ID/2Rg. This slowing down with increasing confinement is consistent with an entropic barrier model which captures conformational entropy loss as macromolecules squeeze through a bottleneck (i.e., array of NPs). To investigate polymer-NP interactions, tracer diffusion of dPMMA is investigated in PMMA:OH-NP. Because of the attraction between dPMMA segments and OH groups on the NPs, tracer diffusion is slower than in the dPS system which involve weak interactions with the phenyl grafted NPs. Third, tracer diffusion is investigated in a bicontinuous structure. By adding NPs that segregate and jam at the interface during phase separation, a novel processing method was developed to prepare discrete or bicontinuous structures of PMMA:poly(styrene-ran-acrylonitrile) (SAN) films. A jamming map of the morphology summarizes the NP concentration and film thickness to produce either morphology. Diffusion of dPMMA in this bicontinuous structure is slower than in PMMA, a result attributed to the tortuosity of the continuous PMMA matrix phase. In summary, macromolecular diffusion has been investigated in PNCs as a function of tracer molecular weight and matrix properties including NP volume fraction, NP size, and NP interactions with tracer molecule. Understanding how to disperse NPs at high loadings is crucial for fabricating polymeric devices including membranes for fuel cells and organic solar cells. These diffusion studies are also practically important for selecting processing conditions to prepare hybrid materials. Finally, the bicontinuous morphology is attractive for applications requiring high interfacial area such as organic solar cells, membranes, catalysis, and fuel cells.