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Photosynthetic water oxidation proceeds at a pentanuclear Mn₄Ca complex bound to amino acid residues in the interior of photosystem II (PSII). It involves the binding of two water molecules, the removal of four electrons and four protons from the Mn-complex/substrate-water entity, O–O bond formation, and dioxygen release. Basic aspects of the reaction cycle of the Mn complex of PSII are discussed: (1) Kok's classical S-state cycle and the corresponding experimental paradigm; (2) recent observations on the (still) enigmatic S₄ state; (3) sequence and characteristics of electron transfer and proton release; (4) a basic nine-step reaction cycle involving eight steps of alternating deprotonation and oxidation of the Mn-complex prior to O–O bond formation and dioxygen release; (5) the energetic constraints of water oxidation in PSII and their mechanistic consequences. In the second part it is considered how the …