In this capability, synchrotron-based x-ray spectroscopies – specifically x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) – combined with techniques for computational spectroscopy are used to determine the electronic structure of a material, and how this structure evolves under operating conditions. For PEC, in situ/operando XAS and XES provide element-specific information on band structure and bending, band edge movement, excition binding energies, and changes in local bonding. Since the XES and XAS probe filled and unfilled states respectively, one can study both the valence and conduction bands, which enables identification of the band gap of a material. Equally importantly, the tunable photon energy at synchrotron beamlines coupled with the elemental specificity of the XAS/XES allows the systematic investigation of multiple materials within a device, leading to the assignment of band edge alignment and how this evolves during operation. Synchrotron radiation also offers the high flux required for study of these materials under the aqueous conditions ubiquitous in photochemical/photocatalytic hydrogen generation technologies. With appropriate cell design, the techniques are readily tailored to accommodate novel cell designs and device architectures (ranging from solid state photocatalysts to colloidal suspensions) as well as environments ranging from highly alkaline to highly acidic. Because one of the biggest challenges lies in the interpretation of the often highly complex XAS/XES spectra, the experiments are tightly coupled with advanced ab initio spectroscopy simulations based on density functional theory (DFT) and the excited core-hole approximation (XCH). The ab initio spectroscopy produces virtual X-ray spectra from model systems that can be directly compared with the in situ/operando data. This iterative experiment-theory approach has been applied to electrochemical and photoelectrochemical systems (under current and past FCTO funding).