The characteristics of superplasticity are discussed from the viewpoint of irreversible thermodynamics, in particular, system evolution and the formation of dissipative structures. A mathematical model combining determinism with indeterminism is used to analyze plastic deformation and superplasticity in metals. Since the plastic deformation of a metal is in essence a dissipative process occurring in a system far away from thermodynamic equilibrium, the instability of the system resulting from structure fluctuations (defects) and thermodynamic bifurcations (possible thermodynamic states) will result in many deformation mechanisms being available to provide the deformation observed in the system. Thus it is virtually impossible to describe all the possible models in conventional terms i.e. by the application of the method of determinism. A new model is proposed, and is shown to account for much of the experimental data reported.