After a few faint stirrings of interest in ion-molecule reactions during the early days of mass spectrometry, the subject was almost completely neglected for many years. There was a renascence of interest in the subject following pioneering experiments by Tal'roze in Russia and independently by Steven­ son and Schissler and Field, Franklin, and Lampe in this country. Since that time, interest in the subject has continued to grow and the number of papers published to expand almost exponentially during the past 15 years. The result is a body of literature that is now almost beyond any single individual's capacity for encompassing in his reading and research activities. Thus, it seemed timely to prepare a book giving in some detail a comprehensive review of the field. Ideally, such a book would be written by one or possibly two authors. Unfortunately, the sheer bulk and variety of the studies of ion-molecule reactions has made it impractical for a single person to prepare a book on the subject in a time that would permit it to be issued before the material covered was obsolete. Consequently, the only practical, although surely not the ideal, solution is to issue a multiauthor volume. The present book represents our best efforts to accomplish this goal. We have secured from 18 authors material representing the most advanced work in the field and the resulting compilation is presented herein.