Transition metal nitrides and oxynitrides are a promising class of functional materials with tunable electronic and optical properties based on anion and cation composition. As such, they offer significant potential for custom-designed applications in photoelectrochemical (PEC) energy conversion. This contribution summarizes our group’s recent advancements and insights into transition metal nitride and oxynitride thin films, emphasizing their potential for solar fuels synthesis. Despite their promise, transition metal nitrides and oxynitrides have been less explored than their oxide counterparts due to complex synthesis requirements. We address these synthetic challenges using non-equilibrium reactive sputter deposition, allowing us to deposit well-controlled thin films within the Ti-Ta-N, Zr-Ta-N, and Zr-O-N composition spaces. Our investigations start with orthorhombic Ta₃N₅, the most established nitride-based photoanode material. With a highly controlled synthesis approach and tailored defect concentrations, we examine the impact of shallow and deep trap states on the operational stability of Ta₃N₅.[1] By comparing water and ferrocyanide oxidation conditions, we observe that shallow oxygen donors within Ta₃N₅ can kinetically stabilize the photoelectrode|electrolyte interface. In contrast, deep-level defects are generally detrimental to its PEC performance. We demonstrate, though, that the concentration of such deep-level defects can be dramatically reduced by controlled Ti doping.[2] While Ti doping minimally affects the structure and band gap of Ta₃N₅, it significantly impacts surface photovoltage, band bending, and photogenerated charge carrier lifetimes. Comprehensive characterization reveals that Ti⁴⁺ ions substitute for Ta⁵⁺ lattice sites, introducing compensating acceptor states and reducing concentrations of deleterious nitrogen vacancies and Ta³⁺ states. This reduction of deep-level defects suppresses trapping and recombination, leading to enhanced PEC activity. Beyond orthorhombic Ta₃N₅, we have explored novel photoanodes with cubic bixbyite-type structures, such as ZrTaN3 and Zr₂ON₂, featuring multiple cation and anion identities. We have recently identified the ternary nitride ZrTaN₃ as a strong visible light absorber and functional photoanode thin film.[3] Complementary density functional theory (DFT) calculations indicate that ZrTaN₃ has a direct band gap that can be tuned by modulating the elemental occupancy of inequivalent cation sites. Finally, changing the anion composition in the Zr-O-N space allows us to tune the band gap in the UV-visible range, achieving PEC activity for oxidation reactions.[4] For crystalline Zr₂ON₂, we observe mild surface oxidation beneficially passivates the surface, while too-thick surface oxides suppress charge transfer and PEC activity. This observation highlights an appealing feature of oxynitrides as photoanodes: the formation of self-passivating surface oxide layers. This passivation makes them suitable for integration with ultrathin atomic layer deposition protection layers, whose major drawbacks are pinholes that can potentially be tolerated by (oxy)nitride semiconductors through the passivating surface oxide layers. Overall, our results underscore the potential of the defect engineering of established and the development of novel transition metal nitride and oxynitride semiconductors for achieving robust and stable materials in solar energy conversion.