Magnesium alloys are highly attractive for their application as structural materials as well as medical implants. A range of alloying systems exists which are investigated, e.g., in terms of alloy microstructure changes, in particular during different processing steps or mechanical testing, and in terms of the associated corrosion performance of the material. Synchrotron radiation and in particular synchrotron radiation microcomputed tomography and nanotomography yield a unique opportunity to investigate such changes and processes in 3D at high resolution and in situ, thus significantly broadening our knowledge base. Herein, the benefits of using synchrotron radiation for the investigation of magnesium alloys with particular respect to its application as a biodegradable implant are demonstrated. Advances in experimental environments for in situ testing are reviewed, and all stages of materials testing are covered in which synchrotron radiation has been used, i.e., from developing and processing of the material, to corrosion testing and assessing implant integration and stability ex vivo. This review incorporates advances both in micro- and nanotomographic imaging regimes and further includes complementary techniques, such as X-ray diffraction, small angle X-ray scattering, X-ray fluorescence, and diffraction tomography. Finally, an outlook into future developments is provided.