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, materials science, etc., it is desirable to publish an advanced series of volumes containing review papers that can be read not only by active researchers in these areas, but also by those who are not experts but who intend to enter the field. The present series aims to serve this purpose. Each review article is written in a self-contained manner by the expert(s) in the area, so that the reader can grasp