The present book is devoted to a rapidly developing field of science which studies the behavior of viscoelastic materials under the influence of deformation~the rheology of polymers. Rheology has long been treated as the theoretical foundation of polymer processing, and from this standpoint it is difficult to overesti­ mate its importance in practice. Rheology plays an important role in developing our ideas on the nature of viscoelastic behavior in connection with the structural features of polymers and composites based on them. This expands the possibilities of employing rheological methods to characterize a variety of materials and greatly magnifies the interest in this field of research. The rheological properties of polymer systems are studied experimen­ tally, chiefly under conditions of shear and tensile strains. One explana­ tion is that many aspects of polymer material processing are associated with the stretching of melts or a combination of shear and tensile strains. In scientific investigations, either periodic or continuous conditions of shear deformation are employed. Each mode provides widespread infor­ mation. In periodic deformation, most attention is generally given to conditions with low deformation amplitudes that do not alter the structure of the polymer system during an experiment (the region of linear deformation conditions). Here the viscoelastic parameters are generally determined with respect to the frequency. Continuous deforma­ tion involves considerable strains, and may be attended by significant reversible and irreversible changes in the structure of a polymer.