The present status of the understanding of water oxidation in photosystem II is reviewed, with special emphasis on theoretical modeling. It is shown that most of the important questions have now been answered. Structures for the intermediate S0 to S3 states of the oxygen evolving complex have been obtained with detailed agreement between theory and experiments. An O–O bond formation mechanism involving an oxygen radical attack on a bridging oxo bond, the oxyl-oxo mechanism, has so far stood all tests by model calculations and spectroscopy. A reasonable understanding of the role of calcium in the OEC and of the Tyrz cofactor has also been obtained. A few minor remaining questions are also discussed.