The effects of water on elastic dissipation in rock under upper crustal conditions are considered briefly in this paper. Experimental evidence indicates that in dry rock the energy dissipation process involves water adsorbed on the surfaces of very thin cracks. In one mechanism the adsorbed water is regarded as a viscoelastic film. Portions of this layer are focused to relax as crack thickness changes with the passing elastic wave. A second possible mechanism involves the relaxation of electrostatic repulsion stresses between oriented polar water molecules on opposing crack faces. When water is present as a bulk fluid, attenuation can be explained by the flow of a viscous fluid into and out of very thin cracks in response to dynamic stresses associated with a passing elastic wave.