In view of the rapid growth in both experimental and theoretical studies of multiphoton processes and multiphoton spectroscopy of atoms, ions and molecules in chemistry, physics, biology, materials science, etc., it is desirable to publish an advanced series of books containing review papers that can be read not only by active researchers in these areas, but also by those who are not experts in the field but intend to enter it. The present series attempts to serve this purpose. Each review article is written in a self-contained manner by experts in the area, so that the reader can grasp the knowledge in the area without too much preparation.