Among others, chemistry, physics, biology, and material sciences have seen a rapid growth in both experimental and theoretical studies of multi-photon processes and spectroscopy of atoms, ions and molecules. This book is an important addition to an advanced series that contains review papers readable not only by active researchers in these areas, but also by those who are intending to enter the field. Written by experts in the area, the reviews are self-contained to allow readers to grasp the key concepts without much preparation. This volume will be useful to active researchers as well as to scientists in biology, chemistry, material sciences, and physics.