To date, ultrafast X-ray spectroscopy has been used to measure electron and hole energies, mid-gap state recombination, small polaron formation, and transport of carriers in multi-layer junctions – explaining its appeal for studying photocatalysis and photoelectrochemistry. While ultrafast x-ray spectroscopy can measure both structural and electronic dynamics, the resulting transient plots are complex and hard to interpret because of how photoexcitation perturbs the core-level transition. We will discuss progress in theory and experiment that now allows routine interpretation of photoexcited X-ray edges using a customized DFT-GW-Bethe Salpeter Equation approach. In addition to interpreting excited state phenomenon, the code can project the X-ray peak structure and the relevant X-ray Hamiltonian components - such as angular momentum coupling, screening, and state-filling – onto the band structure to help give physical intuition as to why the measured spectra appear the way they do. The talk will cover the general principles of how photoexcitation modifies transient X-ray spectra rather than detailed theory. Example experiments will include photoelectrodes ranging from ZnTe to complex transition metal oxides.