Spectroscopic studies can reveal a wealth of information about the rotational and vibrational behaviour of the constituent molecules of gases and liquids. This book reviews the fundamental concepts and important models which underpin such studies, dealing in particular with the phenomenon of spectral collapse, which accompanies the transition from rare gas to dense liquid. Throughout, discussion of the various quantum mechanical and semiclassical theories is interwoven with analysis of experimental results. These include data from optical, NMR, ESR and acoustic investigations. The book concludes with a discussion of the latest theories describing the mechanism of rotational diffusion in liquid solutions. This comprehensive review of theoretical models and techniques will be invaluable to graduate students and researchers interested in molecular dynamics and spectroscopy.