A model of a medium is considered for which the development of microdefects, namely, microcracks, pores and intergrain vacancies, are substantial. Microdefect diffusion and polar phenomena are neglected. The principles of the phenomenological description of the behaviour of such materials are found in damage theories /1, 2/. The relation between the strain history and the strength criterion of a material particle is revealed /3/. The energy aspects of the damage phenomenon were considered /4–7/ in the modelling of fracture waves by strong discontinuities separating materials with different rheological properties. A survey of the further development of the damage theory and its applications to viscoelasticity and creep problems is contained in /8/. The approach used in this paper differs from the traditional approach in two main ways. Firstly, by the axiom of local energy balance and the inequality of dissipation. Following the mechanics of an isolated crack /9/, a term describing the energy flux distributed over a volume that is related to transformation of part of the stored energy and the work of the stresses in the surface energy of the microcracks, is introduced into the local energy balance equation. The dissipation inequality differs from the Clausius-Duhem inequality /10/ by a component that is the dissipation of fracture. The second essential difference is the modification of the principle of macroscopic determinability. It is considered that the running reaction of a material element is a functional determined by the independent prehistories of the strain, entropy, and damage. For a thermoelastic damaged body these fundamental assumptions enable one to clarify two important questions that remain unsolved in the traditional theory of damage: what is the macroscopic interpretation of the measure of material damage and among what group of equations is the equation giving the evolution of damage, the group of rheological relationships or the conservation laws? It is shown that for the medium under consideration, the second rank damage tensor can be Identified with the macroscopic strains of an element unloaded from the running state in a passive process. It is found that damage growth in thermoelastic media is not controlled by the kinetic equation but by a finite relationship connecting the running value of the damage with the running value of the strain, entropy, and the distributed source of the damage. This relationship is a consequence of two thermodynamics principles, i.e., it belongs to the transformed conservation laws. It is shown that within the framework of the assumptions taken for initially isotropic bodies, two and only two kinds of materials are possible whose damage is characterized by either a scalar or a symmetric second-rank tensor. Conditions are formulated which the governing relationships should satisfy so that the dissipation inequality and the invariance requirements would be satisfied in all the allowable processes. The conditions presented significantly narrow down the class of allowable equations of state, especially for a material with a scalar damage characteristic. Assuming small strains and that the rheology is independent of the temperature, the simplest example is constructed of a medium simulating a number of qualitative effects typical for mountain rock /11/.