A rate equation for grain/interphase boundary sliding is developed which is able to accurately account for the deformation of structural superplastics, metallic glasses and nanostructured materials on a common physical basis. III some structural superplastics, however, at the highest strain rates dislocation climb controlled creep becomes important. In its present state of development, the model for the optimal range is able to predict all the three constants of the rate equation ab initio if the grain size is uniform and constant, the grain shape is simple, e.g., rhombic dodecahedron and the number of grain boundaries that participate in a mesoscopic boundary sliding event is known from experiments. When a grain size distribution is present and the grain shape is not regular, the grain size exponent in the rate equation will have to be obtained empirically (in addition to the number of boundaries involved in a mesoscopic sliding event). Understanding of behaviour in the region where grain deformation co-exits with grain boundary flow is phenomenological at present.