The theory presented in this book explains in a consistent manner all dynamic effects (diffusion, neutron scattering, viscoelasticity, optical birefringence) observed in very concentrated solutions and melts of linear polymers from a macromolecular point of view. This monograph reconciles different approaches to polymer dynamics and reflects the modern situation in understanding the relaxation behaviour of the polymer systems. The reader is invited to follow the way from dynamics of a separate macromolecule to the constitutive relations for both weakly and strongly entangled linear polymers. The contents can be related to the fields of molecular physics, fluid mechanics, polymer physics and materials science. This text provides material for one or two semester graduate-level course in polymer dynamics or for graduate students with some background in physics and mathematics. The monograph presents topics in a self-contained way that makes it a suitable reference book for professional researchers in the fields of polymer science, polymer engineering, and materials science.