Usually the shape deformation of a martensitic transformation is so large relative to the stiffness and strength of the surrounding parent austenite phase that plastic accommodation takes place during the growth of a martensite plate. The interfacial structure becomes disorganized loosing its glissile nature and reversion of the transformation requires re-nucleation of the austenite phase. Alternatively, in those instances where the shape deformation is accommodated elastically the interfacial motions take on reversible features. On cooling, the increasing chemical driving force ΔG is counterbalanced by the increasing stored elastic strain energy. On heating, the strain energy plays an essential role in reversing the shape change as well as the motion of such glissile interfaces by working against, and continuously rebalancing, the decreasing chemical driving force. This state of balance between chemical and mechanical forces is termed thermoelasticity. A similar state of balance occurs on mechanical loading of a sample above Ms wherein the thermodynamic contribution of the applied stress is sufficient to make up for the reduced chemical driving force. This force balance is termed pseudoelasticity.