The aim of this volume is to offer a balanced overview of molecular Rydberg spectroscopy as it has developed over recent decades. Recent evolution has split Rydberg spectroscopy into two apparently distinct fields: the one concerns the low (n=3 — 5) Rydberg states, the other the very high (typically n 150) Rydberg states. The former is aimed at spectral levels where Rydberg, valence-shell, and intermediate-type states interact, with a variety of photochemical consequences. The latter considers states extremely close to the ionization limit, from whereionization is possible with a very slight amount of additional energy. Recently developed techniques make it possible to produce ions in well-defined electronic, vibrational and rotational states, including states resulting from spin-orbit or Jahn — Teller splitting. It is then possible to study the structure and reactions of such state-selected ions as well as those of the corresponding neutral molecules. These techniques amount to badly needed high resolution photoelectron spectroscopy. The originality of the present volume consists in giving comparable weight to both low- and high-Rydberg spectroscopies. It aims at providing a glance at the whole of molecular Rydberg spectroscopy in one volume. This involves both theory and experiment, general aspects and treatments of individual molecules. The contributors to this book are among the most prominent protagonists of this double field.