This book presents a quantum framework for understanding inelastic light scattering which is consistent with the classical descriptions of Raman phenomena and Rayleigh scattering, thus creating a unified theoretical picture of light scattering. The Raman effect was discovered in 1928 and has since proved to be one of the most powerful tools to study the molecular structure of gases, liquids, and crystals. The subsequent development of new scientific disciplines such as nonlinear optics, quantum optics,  plasmonics, metamaterials, and the theory of open quantum systems has changed our views on the nature of Rayleigh and Raman scattering. Today, there are many excellent books on the theory and applications of light scattering, but a consistent description of light scattering from a unified viewpoint is missing. The authors’ approach has the power to re-derive the results of both classical and quantum approaches while also addressing many questions that are scattered acrossthe research literature: Why is Rayleigh scattering coherent while Raman scattering is not, although both phenomena are caused by the incidence of a coherent wave? Why are coherent Stokes and coherent anti-Stokes Raman scattering caused by two coherent incident waves both always coherent? This book answers these questions and more, and explains state-of-the-art experimental results with a first-principles approach that avoids phenomenological arguments. Many of the results presented are appearing in book form for the first time, making this book especially useful for young researchers entering the field. The book reviews basic concepts of quantum mechanics and quantum optics and comes equipped with problems and solutions to develop understanding of the key mathematical techniques. The rigorous approach presented in the book is elegant and readily grasped, and will therefore prove useful to both theorists and experimentalists at the graduate level and above, as well as engineers who useRaman scattering methods in their work.