Volume 9 of the Handbook of Magnetic Materials has a dual purpose, as do the preceding volumes in the series. As a textbook it is intended to be of assistance to those who wish to be introduced to a given topic in the field of magnetism without the need to read the vast amount of literature published. As a work of reference it is intended for scientists active in magnetism research. To this dual purpose, Volume 9 of the Handbook is composed of topical review articles written by leading authorities. In each of these articles an extensive description is given in graphical as well as in tabular form, much emphasis being placed on the discussion of the experimental material in the framework of physics, chemistry and material science. Chapter one presents a general account of the magnetism of heavy-fermion systems. Two novel experimental techniques are described in chapters two and five. Chapter two deals with muon spin rotation and chapter five gives an account of the possibilities offered by photon beam spectroscopy. In both chapters it is shown how these sophisticated experimental methods can be used to obtain experimental information not easily obtainable by conventional experimental methods. Chapter three deals with interstitially modified intermetallic compounds of rare earth and 3d elements. Finally chapter four is concerned with thermodynamic approach to phase transitions and shows how the understanding and description of these magnetic phase transitions can be considerably enriched.