Research on superplasticity in metals have clearly established the importance of microstructural evolution combined with several phenomena: (i) dislocation emission and absorption at and near grain boundaries (ii) concurrent grain growth and (iii) grain boundary sliding (GBS) during deformation([1]). The combined evolutionary aspects of these processes is essential to superplasticity mechanism. With current interest in submicrocrystalline metallic alloys produced by severe deformation processes, and their reported large degree of strain hardening under the condition of low temperature superplasticity, it is necessary to critically examine the understanding and modeling of their response in these materials containing subgrains and ultrafine grains. After examining several steady state and evolutionary hardening-recovery models, it has become clear that an existing superplasticity model involving a mix of two mechanisms: glide-climb based deformation in the grain mantle and general dislocation creep within the grain core (proposed in 1994) appear to capture all essential features of the experimental behavior. Data for both microcrystalline and submicrocrystalline materials have been reviewed relative to our model and presented here.