The observed decrease and subsequent increase of the travel time ratio of shear and compressional waves prior to the occurrence of earthquakes in the U.S.S.R. and in the U.S.A. is related to shear dilatancy of rocks around the focal area. Shear-dilatancy is identified as an integral part of elastic and post-elastic deformation of solid isotropic media; it is not, as is widely believed, the result of cracking, but the cause of crack initiation.