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, it is timely to publish an advanced series that contains review papers readable not only by active researchers in these areas, but also by those who are non-experts but who wish to enter the field. This present volume attempts to serve this purpose. Each chapter is written in a self-contained manner by experts in their own area of expertise so that general readers can grasp the knowledge in that area without too much preparation.