A fault plane which has undergone slip over a limited area, a thin intrusion or a crack whose faces have been caused to slide over one another or separate by the action of an applied stress are all physical realizations of a dislocation, that is, an internal surface in an elastic solid across which there is a discontinuity of displacement. Since this discontinuity varies from point to point of the internal surface it is actually a so-called Somigliana dislocation. It can, however, be built up from the more familiar dislocations of crystal physics which have a constant displacement discontinuity. Methods of finding the elastic displacement field around a dislocation in a solid with free surfaces will be outlined. The field of an infinitesimal dislocated area in a semi-infinite solid can be found quite simply, and from it the field of a general dislocation can be obtained by integration. The energy associated with a dislocation is discussed in connexion with energy release in earthquakes.