The justification for combining reports of studies of the rare earths from the disciplines of physics and chemistry can be found in the historical reliance each has had upon the other in the apprehension of these mysterious substances. This was stated in the preface to the first four volumes of the series and is even more true today. For a time, specializations thrived more or less independently but as theory has become more comprehensive it has become possible to progress towards unification of these specializations. Nowhere has this need for a unified approach been more evident than in the study of the rare earths where the insights won in one area become a vision to another resulting in the illumination of the whole. The lanthanides are particularly suited to multidisciplinary study because of the unity derived from the progressive filling of the 4f orbitals by electrons. This introduces many profound physical phenomena such as magnetism as well as the frequently subtle chemical variation that affords fine-tuning of our understanding of nature. Volume 13 of the Handbook continues to promote this interactive approach to the science of the rare earths. Insights won from focused study of some aspect of the field are presented in critical and authoritative reviews. Some of these add important archival material of general and lasting interest. Others on topical subjects will promote new understanding of active areas.