The invention of water splitting by early photosynthetic organisms accompanied by release of molecular oxygen was the cornerstone for the creation of our planet´s oxygen-rich atmosphere and the protective ozone layer in the stratosphere. Both events were essential for the development of higher life on earth. Photosynthesis stores the sun´s energy via CO2 reduction in form of energy-rich organic compounds. It is the only basic source of food and other biomaterials on earth and has created all our fossil fuels. The catalytic water oxidation reaction is performed by a protein-bound Mn4OxCa cluster located in the so-called photosystem II (PSII) [1]. The cofactor´s reaction cycle is comprised of 5 distinct redox intermediates Sn, where the subscript indicates the number of stored oxidizing equivalents in the Mn cluster (n = 0 to 4) required to split two water molecules and release one O2. A redox-active tyrosine residue couples the fast light-induced single-electron charge separation to the slow catalytic four-electron water oxidation process. All these states are paramagnetic; thus electron paramagnetic resonance is the method of choice to study the catalytic cycle and understand this important biological process. The S-states are trapped by laser flash/freeze quench techniques and their electronic structure is studied by advanced EPR techniques (ENDOR, ESEEM, ELDOR-detected NMR) [2]. The experimental data are corroborated by DFT calculations. The results give information on the electronic structure (spin and oxidation states) of the manganese ions [3], the function of the Ca2+ [4], the effect of the amino acid surrounding as well as the binding, location and reaction dynamics of the substrate water [5], the O-O bond formation and the release of molecular oxygen [6]. A robust model for the water oxidation mechanism is derived. This information can be used for the design of bioinspired catalysts for water oxidation.