CONTENTS 1. Introduction 4175 2. Photosystem II 4176 2.1. Electron Transfer Chain 4176 2.2. Oxygen-Evolving Complex and the Mn 4 Ca Cluster 4177 2.3. X-ray Radiation Damage to the Metal Cluster 4179 3. Structural Changes of the Mn 4 CaO 5 Cluster during the Catalytic Cycle 4179 3.1. Dark Stable S 1 State Structure 4179 3.2. Structural Changes of the Mn 4 CaO 5 Cluster 4180 3.2.1. S 1 to S 2 (g = 2, MLS State) Transition 4181 3.2.2. S 1 to S 2 (g = 4 State) Transition 4181 3.2.3. S 2 to S 3 Transition 4181 3.2.4. S 3 to S 0 Transition 4182 3.2.5. S 0 to S 1 Transition 4182 3.3. Structural Changes and Calcium 4182 3.3.1. Mn XAS and Ca or Sr 4183 3.3.2. Mn XAS and Ca Depletion 4183 3.3.3. Ca or Sr XAS 4184 3.3.4. Orientation of Ca/Sr−Mn Interactions in the PS II Membrane 4185 3.3.5. Changes of the Ca−Mn Distances During the S-State Transitions 4185 3.4. Ligands of Mn and Ca and Site-Specific Mutants 4186 3.5. OEC in Monomeric and Dimeric PS II from Cyanobacteria 4186 4. Electronic Structure of the Mn 4 CaO 5 Cluster 4187 4.1. Electronic Structure Probed by X-ray Spectroscopy 4188 4.2. Mn K-edge Absorption Spectra of PS II 4190 4.2.1. Comparison of OEC from Plants and Cyanobacteria 4190 4.3. Mn X-ray Emission Spectra of PS II 4191 4.3.1. Kβ 1,3Transitions 4191 4.3.2. Resonant Inelastic X-ray Scattering 4192 4.4. S-States and Summary of the Oxidation States 4194 4.5. Lessons from XES of Model Complexes about the Intermediate States 4194 4.5.1. High-Spin and Low-Spin Mn V with Kβ 1,3 / Kβ″ Spectra 4194 4.5.2. XES for Direct Detection of Oxo-Bridges Using Kβ 2,5 /Kβ″ Spectra 4194 4.5.3. Protonation State of the Oxo-Bridges with Kβ 2,5 /Kβ″ Spectra 4195 5. Current Understanding of the Water-Oxidation Mechanism 4197 6. Toward an Understanding of Protein and Chemical Dynamics of PS II 4198 7. Conclusions 4200 Author Information 4200 Corresponding Authors 4200 Notes 4201 Biographies 4201 Acknowledgments 4201 References 4201