Published by the American Geophysical Union as part of the Geophysical Monograph Series.The following chapters were presented as a tutorial and as invited papers at the Chapman Conference held in Sendai/Mt. Zao, Japan, October 12-16, 1987. The authors were chosen both for their expertise in their subject matter and also for their willingness to address an audience of graduate students and newcomers to the field. We hope they have succeeded. The contributed papers presented at the conference were published in a special section of the January 1989 issue of the Journal of Geophysical Research, Space Physics.