Among the many tools in material science, electronic spectroscopies, particularly those utilizing synchrotron radiation, are of great interest. Techniques such as spin-resolved photoemission, X-ray magnetic circular and linear dichroism, resonant X-ray scattering, and X-ray magnetic microscopy offer unique insights into magnetic phenomena and materials. However, these advancements require robust theoretical support. The ab-initio theoretical investigations presented here utilize a one-electron picture within Density Functional Theory, employing multiple scattering theory to address the electronic structure problem. The spectroscopies discussed are largely influenced by spontaneous magnetization and relativistic effects, especially spin-orbit coupling. To comprehensively address these effects, a fully relativistic formalism is employed. One key objective is to apply the Korringa-Kohn-Rostoker (KKR) band structure method in its relativistic form across various spectroscopies, with results compared to available experimental data. The first chapter introduces the theoretical foundation, while the second focuses on photoemission spectroscopies, including spin-resolved Auger electron spectroscopy and Fano effects. The third part explores resonant magneto-optical effects in the X-ray regime, and the final chapter examines non-collinear spin structures within multiple scattering theory, detailing its application to photoemission and X
