Results are presented of an investigation of low-frequency relaxation processes in concentrated solutions and melts of polymers -- systems of weakly coupled linear macromolecules. The effectiveness of a single-molecule approximation is demonstrated in which it is assumed that each macromolecule moves among its neighbors as if in an after-acting medium. An approach is examined which is confirmed by studying phenomena of optical anisotropy, diffusion, and neutron scattering, and which leads to a theory of viscoelasticity of polymer systems that turns out to be self-consistent.