The ten invited papers presented at the Symposium on The Role of Ion Composition in Understanding Magnetospheric Processes, which was held on 11 August 1981 in Edinburgh, Scotland, have formed the principal basis for this book. The symposium was a part of the Fourth Scientific Assembly of the International Association of Geomagnetism and Aeronomy and was convened by the Division III Working Group on Composition of Hot Magnetospheric Plasma under the co­ chairmanship of Dr. Gerhard Haerendel and myself. The symposium papers have been expanded and are supplemented in this book by several additional papers to provide a more comprehensive topical review and a current status report on experimental and theoretical research that relate to our present understanding of the energetic ion composition in Earth's magnetosphere. Thus, the papers include new results as well as a review of published results, with rather extensive reference lists, in the hope that this collection will be useful both for information on current research activities as well as a somewhat lasting topical resource for the space science community.