Thermosets are a ubiquitous class of polymeric materials used due to their excellent mechanical properties and stability resulting from their covalently cross-linked architecture. Unfortunately, these covalent cross-links preclude the reshaping or reprocessing these materials, which limits both the technological applications and sustainable use of these materials. Recent efforts have focused on the incorporation of dynamic covalent bonds into cross-linked materials to enable their continuous reprocessing (Chapter 1). We have focused on applying these strategies to cross-linked polyurethane networks, since these are the most prevalent class of cross-linked polymers. We first demonstrated that the synthesis of cross-linked polyhydroxyurethanes (PHUs) generates cross-linked resins that are inherently reprocessable, likely through mechanically-activated transcarbamoylation reactions that occur at elevated temperature (Chapter 2). We further studied how the polymer structure affects the thermal stability and reprocessability of PHUs (Chapter 3). Due to the slow and incomplete reprocessing observed in these systems, we incorporated more dynamic disulfide bonds into PHU networks as one strategy to enable rapid, quantitative reprocessing of these materials (Chapter 4). Through our many studies on the reprocessing of these and related materials, we have discovered that cross-linked polyurethane resins are inherently dynamic under mild conditions in the presence of certain catalysts, and we demonstrate the potential to use this finding to reprocess PUs traditionally considered to be thermoset materials (Chapter 5).