A problem formulation for a continuum thermomechanical description of martensitic phase transitions (PT) in elastoplastic materials is presented. Stress history dependence, during the transformation process, is a characteristic feature of the new PT criterion. Relatively simple mechanical models for noncoherence and fracture at interfaces are proposed. Solution algorithms (which include, in particular, the solution of standard elastoplastic contact problem) and numerical results for elastoplastic model problems with PT (noncoherent interface, interface with fracture, moving interface, progress of PT zone) are presented. It is shown that: (a) a noncoherent interface and fracture promote considerably nucleation ; (b) a noncoherent interface has low mobility or cannot move at all which agrees with known experiments ; (c) for elastic materials the growth of a single connected region of new phase occurs ; for elastoplastic materials complex multiple connected PT region (discrete microstructure) is obtained.