Magnesium alloys have large potential in orthopedic and dental applications because their Young's moduli are similar to that of human bones and their natural biodegradability obviates the need for a follow-up surgery. However, it is crucial to control the degradation rate to maintain mechanical support for a long enough time for tissues to heal and avoid cytotoxic effects arising from uncontrolled Mg dissolution and hydrogen evolution. Plasma-based techniques such as sputtering, cathodic arc deposition, and plasma immersion ion implantation are very useful. Different from other common biometals such as stainless steels and titanium alloys, the biodegradable surface of Mg alloys is dynamic. As the metal degrades or dissolves, a new surface or interface with the biological medium forms constantly and so in conducting surface modification, the balance among degradation rate, mechanical integrity, and biocompatibility is very important. In this invited talk, some of our recent results will be presented.