Hydrogels are ubiquitous synthetic and biological polymeric networks, prominent in a range of industrial applications and commonly found in the natural world. The applications of hydrogels to solve global challenges including nanoscale filtration to create potable water and controlled drug delivery for increasingly advanced medical interventions requires an understanding the dynamics of nanoparticles and other molecules within hydrogel networks. For this dissertation, I studied nanoparticle dynamics in synthetic hydrogels using single particle tracking. By tuning both the hydrogel nanostructure and nanoparticle probe characteristics, I investigated how the addition of immobile nanoparticles in nanocomposite hydrogels, probe shape and size, and the mesh size of the network affect the dynamics of individual nanoparticles. Single particle tracking allowed for the visualization of single nanoparticle trajectories, which were then analyzed to yield quantitative information including the mean squared displacement, the van Hove distribution function, and the non-Gaussian parameter. In silica-polyacrylamide hydrogel nanocomposites, the interactions between the poly(ethylene glycol) (PEG) functionalized nanoparticles and the static silica nanoparticles dominated dynamics, as the presence of interactions could be tuned by changing the pH, affecting nanoprobe mobility. During the gelation of a nearly homogeneous tetra-PEG hydrogel with a final mesh size similar to the probe diameters, nanorods of comparable minor diameters to their spherical counterparts exhibited enhanced dynamics as the hydrogel network formed. This suggests that nanorods under increasing confinement exhibit distinct mechanisms of mobility, exhibited by the presence of caging to mobile transitions. Network defects also have a pronounced effect on probe dynamics, though the extent is dependent on the nanoparticle size compared to the length scale of the defects as measured in defective tetra-PEG hydrogels. For example, smaller probes exhibit increased dynamics due to a larger percentage of mobile particles as they are more sensitive to small perturbations in the network. Identifying factors that impact nanoparticle dynamics in swollen networks at the nanoscale, including probe-particle interactions, probe shape and size, and network defects will inform the thoughtful design of hydrogels for applications such as drug delivery systems and filtration membranes.