The replacement of fossil fuels by hydrogen generated by water splitting with sunlight is a very attractive solution to today’s energy problem. In order to develop a practical technology based on these elements, robust and efficient water oxidation catalysts (WOCs) need to be developed. The molecular complexity involved in water oxidation is such that sophisticated catalysts consisting of transition metal complexes – which can access low-energy reaction pathways – are considered essential. Complexes based on first row transition metals such as Mn, Co, and Fe have been reported recently, in addition to those of the second row Ru and third row Ir, which had been previously described. In the present chapter, a comprehensive view of these WOCs and their performance is offered.