This book contains the lectures presented at the Summer Advanced Study Institute, 'Earth's Particles and Fields' which was held at the University of Sheffield, England, during the period August 13-24, 1973. One hundred thirty nine persons from sixteen different countries attended the Institute. The authors and publisher have made a special effort for rapid publication of an up-to-date status of the particles, fields, and processes in the Earth's magnetosphere, which is an ever changing area. Special thanks are due to the lecturers for their diligent preparation and excellent presentations. The individual lectures and the published papers were deliberately limited; the authors' cooperation in conforming to these specifications is greatly appreciated. The contents of the book are organized by subject area rather than in the order in which papers were presented during the Institute. Many thanks are due to Drs Rolf Bostrom, J. Ronald Burrows, Robert W. Fredricks, Thomas R. Kaiser, Bernt N. Maehlum, Christopher T. Russell, and Martin Walt who served as session chairmen during the Institute and contributed greatly to its success by skillfully directing the discussion period in a stimulating manner after each lecture.