Oxygen evolution by photosynthetic water oxidation has shaped life on planet Earth. This unique biological reaction may provide important clues for developing artificial devices (‘artificial leaves’) for splitting water into O2 and H2 by visible light. In this chapter, we summarize the latest structural and mechanistic information on photosystem II, its water-oxidizing complex (WOC), and O2 formation from water at its Mn4CaO5 cluster. Structural information on the WOC derived by X-ray crystallography, X-ray spectroscopy, EPR/ENDOR, and theoretical approaches are compared and discussed in detail. After a description of the basic mechanism of water-oxidation (Kok model), our knowledge about the storage of oxidizing equivalents, substrate water-binding, the release of the products O2 and H+, and the energetics of water-oxidation is described. The near electroneutrality of the water-splitting process via coupling of electron and proton transfer steps and the stabilization of the initial light-driven charge separation by spatial separation and heat dissipation are emphasized as important design principles for coupling fast photochemistry with comparatively slow chemical reactions. The chapter concludes with a discussion of recent DFT-based mechanistic proposals for photosynthetic water oxidation, and with an attempt to identify lessons for the design of artificial systems.