A review of previous work concerning the influence of water on the wear of alumina, silicon carbide, silicon nitride (or sialon) and zirconia (or partially stabilized zirconia, PSZ) has revealed a very confused picture: wear rates can either be increased or decreased by the presence of water. In an attempt to rationalize this behaviour, experiments are described in which the above ceramics are worn against a bonded diamond counterface in various environments - air, water, a synthetic diester (di-2-ethylhexyl sebacate) and a diester containing dissolved water. All of the observed trends in wear can be explained in terms of the competing effects of water-accelerated crack growth leading to increased microfracture, and tribochemical reactions leading to reduced microfracture. The relative importance of these two processes depends on the specific ceramic/environment combination and on the counterface roughness.