We demonstrate that one can eliminate the variables related to the displacement from the system of equations of state or of motion. The latter condenses into a single equation solely affecting the order parameter. The elimination of elastic variables is fundamental. It lies in the foundation for all subsequent discussions in this book. We show that if a process zone exists, the order parameter induces a localized spontaneous strain within the zone. This, in turn, generates a strain field throughout the solid. Two primary elastic fields influence the daughter phase embedded within the mother phase matrix. The first originates from stress concentrators such as cracks or dislocations. The daughter phase itself generates the second. We also prove that one can eliminate the strain induced by the process zone from the equations of motion and the equations of state. As a result, one describes the process zone by a single nonlinear equation with the solitary dependent variable, the order parameter. This equation depends only on the trace of the strain tensor of the undressed defect. The latter is known from linear elastic mechanics. The solution to this equation provides complete information on the dynamics and statics of the process zone. Henceforth, in the following, we focus exclusively on these equations to describe process zones.