The nine review articles and roughly 30 contributed papers contained in this volume survey the present understanding of the molecular motion of polymers in concentrated solutions, melts and the amorphous state. Although the main emphasis is on results from neutron scattering experiments, about half of the contributions relate to complementary techniques such as light scattering and NMR. The book highlights three areas of active reseach that have received increasing attention in recent years. First, work on polymer relaxation near the glass transition and in glassy systems has shown that the application of different experimental techniques is indispensable due to the broad range of time scales over which these phenomena occur on a molecular level. Second, it is reported that the internal Brownian motion of long chain molecules and their diffusion in an entangled environment still does not have a generally accepted physical picture, although detailed insights into the microscopic motion have been obtained. Third, important progress has been made in unravelling the characteristics of phase separation of polymer blends, in which modifications to the ordinary laws of diffusion are of particular interest.