The aim of the authors in this monograph has been to present a general review of the magnetic properties of the rare earth metals, and a comprehensive, unified, and coherent account of the magnetic structures and excitations. These both reflect the nature of the fundamental interactions, and determine many of the characteristic properties of the metals. The authors have tried to concentrate on the essential principles and their applications to typical examples, generally restricting the discussion to the pure elements and considering alloys and compounds only when they are necessary to illuminate particular topics. Both authors have been involved for some time in the effort that has been made in Denmark to study, both theoretically and experimentally, the magnetic structures and especially the excitations in the rare earths. This account of the subject represents the results of their experience, and it is has been written in the hope that it will be useful not only to those who have a special interest in rare earth magnetism, but also to a wider audience who wish to learn something about the techniques and achievements of modern research in magnetism.