A unified description of martensitic pseudoelasticity based on the Laves-Cahn model of deformation pseudotwins is presented. In particular, we treat the puzzling problem of "rubber elasticity" in disordered alloys. The dynamics of twin-boundary motion reflects the nucleation and growth characteristics of the decay of a metastable state. The model is amenable to direct experimental test by time-dependent x-ray diffraction. © 1984 The American Physical Society.