Abstract
The author reviews the basic elements of the theory of X-ray absorption using the tools provided by the theory of multiple scattering. He first uses an approximate momentum space approach which gives a very clear physical insight where the final formulas expressing EXAFS and XANES, i.e. the structures appearing in the absorption coefficient above the edge of a deep core-level threshold, are given in terms of eigenstates of the photoelectron momentum. He then reviews the correct formalism, the curved-wave theory for which he needs to work in angular momentum space. Simple graphic representations are given for the multiple-scattering function.