First thoughts of writing a textbook on X-ray spectroscopy followed the publication of a review article on resonant inelastic X-ray scattering (Ament et al., 2011). Jeroen van den Brink and I set up an outline of what basis was necessary to read the review article. The idea was that it should be accessible to graduate students or scientists new to the field with little prior knowledge of spectroscopy with an emphasis on the theoretical background. Since it is impossible to interpret spectroscopy without a model of the system that is being studied, it is also necessary to consider a certain amount of atomic and condensed-matter physics. The focus of the book is predominantly on materials that are strongly correlated, i.e. the interactions between the electrons are usually larger than the bandwidths. The book does not aim for completeness in theoretical approaches, experimental overview, or bibliography. Rather it aims to give the reader a basis for further study and an overview of the necessary ingredients to interpret X-ray spectra.

The book is divided into the following chapters. The first chapter gives a brief overview of what is needed to interpret a spectrum. Since the absorption and scattering of X-rays is, to a first approximation, a local process, Chapters 2-4 lookat the local electronic structure starting from atomic physics and introduce solid-state effects by crystal fields. These sections rely heavily on group theory necessary to understand the complex interactions between electrons in the atomic orbitals and the X-ray photons that carry angular momentum through the polarization vectors. Obviously, we can only skim the surface of these topics which are by themselves the subjects of entire books. Chapter 5 then discusses many-body effects, focusing on the description of the Coulomb interactions in terms of linear and angular momentum. The latter in particular is responsible for many characteristic features in the X-ray absorption spectra known as multiplet structures. Chapter 6 describes the interaction between the photons and the electrons.