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.