This monograph contains the fundamentals of the theory and gives a compact and consistent picture of the different relaxation phenomena in very concentrated solutions and melts of linear polymers (diffusion, neutron scattering, viscoelasticity, and optical birefringence) from a macromolecular point of view and without any specific hypotheses. It can be considered as complementary reading to the classic textbook by Doi and Edwards. The original approach taken to the problem allows us to understand why numerous attempts to find the 3.4-index law for the viscosity coefficient of linear polymers in the frame of the reptation-tube model were doomed to fail, and have failed during the last twenty years. It also helps us to derive the proper constitutive relation for polymers of different architecture.The monograph can be used as a textbook for graduate students with some background in physics and mathematics. It could provide material for a one- or two-semester graduate- level course in polymer dynamics. The monograph presents topics in a self-contained way that makes it a suitable reference book for professional researchers in the fields of rheology, polymer science, polymer engineering, and material science.