This 1998 book was the first to describe ZEKE (Zero Kinetic Energy) spectroscopy, a high resolution spectroscopy of molecular ions, neutral short-lived intermediates, and other species. The author's approach is to use a minimum of equations with large numbers of figures to help the reader towards a basic understanding of the many unique concepts of this form of spectroscopy. Since 1984 ZEKE spectroscopy has matured into a very high resolution spectroscopy for the study of cations, anions and, indirectly through these species, of neutrals, including very short-lived intermediates in chemical reactions. It has even yielded the first direct spectroscopic data on elusive transition states of chemical reactions. This book will be of interest to anyone interested in the spectroscopy of ions or neutrals, particularly short-lived neutrals, and to reaction kineticists interested in the study of reactions involving such highly state-selected species.