This dissertation presents new research on the field-dependent surface resistance of nitrogen-doped niobium. We prepared a set of single-cell 1.3 GHz niobium accelerator cavities with nitrogen doping and measured the surface resistance of these cavities. We studied the relation between the field-dependent reduction in the surface resistance (the "anti-Q-slope") and the doping level, finding a strong empirical link between high nitrogen content and the suppression of quasiparticle overheating. We prepared another set of cavities with nitrogen infusion and studied their surface resistance, finding similar behavior to that of the nitrogen-doped cavities despite the stark differences in impurity content. We found evidence suggesting that the anti-Q-slope is a surface effect, depending on the surface concentration of nitrogen and sensitive to surface contamination. We developed a new thermal modeling framework for studying local models of the surface resistance in superconducting niobium cavities with depth-dependent material parameters. We assessed several recent models of the anti-Q-slope, finding that none provide a fully satisfying explanation of the behaviors observed in experiment. Finally, we developed, constructed, and began commissioning the DC Field Dependence Cavity, a new apparatus for investigating the field-dependent surface resistance of nitrogen-doped niobium and other materials under superposed DC and RF surface magnetic fields.