Among the many tools one has in hand in material science, the various electronic spectroscopies are of particular interest. Techniques based on synchrotron radiation, such as spin-resolved photoemission, X-ray magnetic circular and linear dichroism, resonant X-ray scattering and also X-ray magnetic microscopy have been shown to provide unique tools for the study of the magnetic phenomena and magnetic materials. However, it became apparent that all these new achievements need a strong theoretical support. Ab-initio theoretical investigations presented here adopted a one-electron picture, within the framework of Density functional theory, using the multiple scattering theory to solve the electronic structure problem. Most of the spectroscopies to be dealt with here can be seen as a direct consequence of the presence of a spontaneous magnetisation and relativistic effects, in particular the spin-orbit coupling. To deal with all relativistic effects and magnetism on the same level, the fully relativistic formalism will be used in the following. Thus, one of the goals of this work is to apply the Korringa-Kohn-Rostoker (KKR) band structure method in its relativistic version to a wide range of spectroscopies. Accordingly, all results will be compared with available experimental data as far as possible. Based on these results various aspects connected with experiments will be discussed in detail. In the first chapter the theoretical basis of this work will be introduced. The second part of this thesis is dedicated to the photoemission spectroscopies, i.e. spin-resolved Auger electron spectroscopy, Fano effects in valence band photoem ission and core level photoemission. In a third part we discuss resonant magneto-optical effects in the X-ray regime. Last chapter is dealing with non-collinear spin-structures within multiple scattering theory and its application to studies of photoemission and X-ray absorption is described.