A large variety of nonlinear spectroscopic methods have so far been developed and applied to studies of molecular structure and dynamics. Some of these methods utilize complex techniques, making it difficult to understand the underlying principles. Furthermore, most of these methods are referred to by nicknames or acronyms, which keeps them shrouded in mystery for most non-specialists who are thereby discouraged from mastering and applying the methods to molecular systems. Under these circumstances it was judged appropriate and timely to overview those nonlinear spectroscopic methods which are currently particularly useful for studies of molecular structure and dynamics. Thus, the Commission on Molecular Structure and Spectroscopy of the International Union of Pure and Applied Chemistry decided to publish a book, intended to provide an introduction to nonlinear spectroscopy for non-specialists.