During the last four years, major progress has been made in mapping regional variations in the strength of the lithosphere and in understanding the rheological properties of the upper mantle which are responsible for the plate‐like behavior of the lithosphere. It has long been recognized that the apparently rigid translations of the plates and the support of large topographic loads on the earth's surface require a strong lithosphere overlying a weak asthenosphere. Recently, interest in defining the detailed mechanical properties of the lithosphere has greatly increased, perhaps stimulated by the discovery of a double Benioff zone beneath Japan which may be related to the unbending of the subducted oceanic plate. Many investigators seem to be reaching a general consensus on a model of the oceanic lithosphere which is consistent with thermal models of the upper mantle, experimental rock mechanics, gravity and bathymetrie surveys, and patterns of seismicity and focal mechanisms.