An interesting route to supplying renewable energy on a large scale is the direct conversion of solar energy into chemical fuel. Perhaps the simplest system one can imagine is the direct photocatalytic dissociate of water on a semiconductor surface to produce hydrogen and oxygen. Titania has a remarkable, but not unique, propensity to support such photocatalysis. We have used DFT calculations to identify the important surface structures, reaction sites and inter-molecular interactions that govern the photocatalytic reaction.