This volume is the latest volume in a series that is a pioneer in compiling review articles of nonlinear interactions of photons and matter. It has made an essential contribution to the development and promotion of the related research fields. In view of the rapid growth in multi-photon processes and multi-photon spectroscopy, care has been taken to ensure that the review articles contained in the series are readable not only by active researchers but also those who are not yet experts but intend to enter the field.