The book covers the quantum mechanical fundamentals of molecular spectroscopy from the view of a practicing spectroscopist, rather than a theoretician. Therefore, the book provides the background and derivation of the subjects necessary to understand spectroscopy: stationary energy states, transitions between these states, selection rules and symmetry. Several forms of spectroscopy, used in many fields of science, are discussed, such as fluorescence, surface spectroscopies, linear and non-linear Raman spectroscopy and spin spectroscopy.