"The last decades have been characterized by great advances in experimental and theoretical physical methods which find application in the Infrared Molecular Spectroscopy. As a consequence, several recent monographs and books have evidenced the most advanced aspects of Molecular Spectroscopy, such as non-linear effects, dynamics of floppy molecules and Van der Waals complexes, and sophisticated theoretical and laboratory methods. On the other side, the evolution of the knowledge in this field makes it desirable to go a few steps back, thinking of an updated detailed formulation of the elementary basic principles and rules of the traditional linear absorption spectroscopy of semi-rigid molecules, which is preliminary to the most advanced developments, in line with the latest theoretical developments. This is even more desirable, if one thinks of the increasing need for people able to handle vibration-rotation spectra, with the present wide potentiality of applications in the atmospheric and planetary research"-- Provided by publisher.