In view of the rapid growth in both experimental and theoretical studies of multi-photon processes and multi-photon spectroscopy of atoms, ions and molecules in chemistry, physics, biology and materials science, this volume is part of a series which contains review papers that can be read not only by active researchers in these areas, but also by those who intend to enter the field. Each review article is written in a self-contained manner by the expert(s) in the area, so that the reader should be able to grasp the knowledge without too much preparation.