Nanostructured heterogeneous catalysts will play a key role in the development of robust artificial photosynthetic systems for water photooxidation and CO 2 photoreduction. Identifying the active site responsible for driving these chemical transformations remains a significant barrier to the design of tailored catalysts, optimized for high activity, selectivity, and lifetime. This highlight reveals how select recent breakthroughs in the application of in situ surface and bulk X-ray spectroscopies are helping to identify the active catalytic sites in a range of liquid and gas phase chemistry.