The process zone (PZ) that surrounds and precedes a crack is a common feature of fracture in engineering polymers. Depending on the material, the specimen geometry, the temperature, and the loading conditions various types of microdefects such as crazes, shearbands, microcracks, micro-voids, etc, constitute the process zone. The microdefects are formed in response to stress concentration, and shield the crack tip from high stress level. There is a complex crack – damage interaction, which is briefly addressed by means of a semi-empirical method. On a continuum mechanics level, the PZ appears as a domain with effective elastic properties different from that of the original material. The crack and PZ evolve as one system with multiple degrees of freedom. It is regarded as a Crack Layer (CL) in contrast with the conventional image of crack as an ideal cut. There are thermodynamic forces responsible for CL growth, which are defined as derivative of Gibbs free energy with respect to the corresponding CL “coordinates”. The thermodynamic forces can be expressed as integrals of the Energy Momentum Tensor of elasticity. Onsager type relations between CL growth rates and corresponding CL forces constitute a system of constitutive equations for CL propagation. Examples of solution of these equations, and comparison with experimental data as well as with conventional models are presented in accompanying paper.