The incorporation of nanoparticles into polymer networks can lead to improvements in current technologies and solutions for global challenges. For example, mechanical properties can be enhanced while maintaining processability, controlled release of drugs can be enabled, and desalinization membrane capabilities can be improved. Inherent to these applications is controlling the localization and mobility of nano-scale diffusants. To improve the functionality of polymer nanocomposite materials, a fundamental understanding of how polymer parameters affect nanoparticle diffusion is required. For this dissertation, I studied nanoparticle diffusion within heterogeneous, homogenous, and biological polymer networks. Using single particle tracking, I investigated the role of confinement, heterogeneity, extent of gelation, and swelling on nanoparticle mobility. For each study, nanoparticles were integrated into the polymer network, hundreds of trajectories were followed in fast frame rate videos, and then these trajectories were analyzed to yield quantitative information such as mean-squared displacement, displacement distributions, and diffusion coefficients. From quantitative analysis of nanoparticle dynamics, insight into the impact of various polymer properties on mobility were obtained. Specifically, in two environmentally responsive, heterogeneous hydrogels, polyacrylamide and poly(N-isopropylacrylamide) gels, confinement, dynamic heterogeneity, and enthalpic interactions impacted nanoparticle mobility, which varied from diffusive to localized. Solvents and temperature were used to elicit collapse of these heterogeneous networks, which resulted in drastically decreased nanoparticle mobility. In these studies, displacement distributions revealed the extent of Gaussian behavior which was found to be a marker of environmental heterogeneity or cage size. For gelation of tetra-poly(ethylene glycol) networks, a proposed homogeneous network, polymer concentration and extent of gelation impacted nanoparticle diffusion. Using analytical tools developed for synthetic polymer networks, nanoparticle diffusion within the cell cytoplasm was studied, revealing that nanoparticles moved further and faster in cancerous cells and in cells with disrupted actin networks. Overall, our studies revealed that confinement alone does not control nanoparticle mobility within polymer networks and that dynamic heterogeneity played a significant role. Identifying factors which determine how nanoparticles diffuse in polymer networks will aid in the design of novel and existing membranes, scaffolds, and drug delivery systems.