A thermodynamic theory of heterophase structures is proposed, describing the relation of the volumes, shape, internal structure, and mutual disposition of the crystals that are formed in a phase transformation in the solid state. An important feature of solid heterophase systems is the presence of internal-stress fields arising as a consequence of the contact between the phases. The tendency to reduce the elastic energy of the internal stresses leads to the breakdown of the crystals composing the heterophase system into elastic domains and to the formation of ensembles of crystals arranged in an ordered manner. As a result of the competition between the elastic energy of the internal stresses and the interphase surface energy in a solid, metastable many-phase states arise, characterized by a regular spatial distribution of plate-like structural elements.