There is an urgent need to transition from fossil fuels to solar fuels — not only to lower CO2 emissions that cause global warming, but also to ration fossil resources. Splitting H2O with sunlight emerges as a clean and sustainable energy conversion scheme that can afford practical technologies in the short to midterm. A crucial component in such a device is a water oxidation catalyst (WOC). These artificial catalysts have mainly been developed over the last two decades, which is in contrast to Nature’s WOCs, which have featured in its photosynthetic apparatus for more than a billion years. This time period has seen the development of increasingly active molecular WOCs, the study of which affords an understanding of catalytic mechanisms and decomposition pathways. This Perspective offers a historical description of the landmark molecular WOCs, particularly ruthenium systems, that have guided research to our present degree of understanding.