Publisher Summary This chapter presents the creep constitutive equations for damaged materials. Constitutive equations used to describe the plastic behavior of materials have largely been developed intuitively, the main concern being that the equations should describe the material macroscopic properties. A general thermodynamic description of material behavior in terms of internal state variables that are a measure either of the present dislocation structure, or of the distribution and size of voids within the material is explained. This equation of state can only be obtained by postulating a reversible process by which the present state could be reached. The free energy is calculated by following this path to the present state. There may be a large number of such reversible paths, and the one chosen need not parallel the actual path that is followed during the irreversible process. The state of the material can be described in terms of the volume fraction of voids that it contains. If it is assumed that all the voids are the same size and are uniformly spaced along the boundary, then one can further isolate an element of material of volume, which contains a single void, and assume that it experiences the stress applied to the grain-boundary element. The void growth by grain-boundary diffusion is also elaborated.