This mini-review summarizes my postdoctoral research in the labs of T. Wydrzynski/C.B. Osmond, J.H.A. Nugent/M.C.W. Evans and V.K. Yachandra/K. Sauer/M.P. Klein. The results are reported in the context of selected data from the literature. Special emphasis is given to the mode of substrate water binding, Mn oxidation states and the structures of the Mn cluster in the four (meta)stable redox states of the oxygen evolving complex. The paper concludes with a working model for the mechanism of photosynthetic water oxidation that combines CL-oxo bridge oxidation in the S-3 State (V.K. Yachandra, K. Sauer, M.P. Klein, Chem. Rev. 96 (1996) 2927-2950) with O-O bond formation between two terminal Mn-hydroxo ligands during the S-3 --> (S-4) --> S-0 transition. (C) 2000 Elsevier Science B.V. All rights reserved.