Biological water splitting – the process via which plants use sunlight to generate fuel (sugars) – underpins all life on earth. It is performed by a single cofactor in nature, a tetramanganese-pentaoxygen calcium cluster embedded in the Photosystem II supercomplex. Using Electron Paramagnetic Resonance (EPR) spectroscopy and related techniques we have been able to identify the sites of substrate water binding and important structural changes which facilitate substrate binding. We are now poised to examine the formation of the last intermediate and the mechanism of O-O bond formation.