Degradable metals are breaking the current paradigm in biomaterial science to develop only corrosion-resistant metals. In particular, metals which consist of trace elements existing in the human body are promising candidates for temporary implant materials. Such implants would be needed for a short time to provide mechanical support during the healing process of the injured or pathological tissue. Magnesium (Mg) and its alloys have been investigated recently by many authors as a suitable degradable biomaterial. The degradation of Mg alloys in vivo has been investigated in several animal studies. The findings of these studies will be critically discussed and related to Mg corrosion principles. In contrast to slow corroding metals the designated complete degradation or corrosion of Mg alloys is conjunct with the limited use of the standard procedure for biocompatibility testing (ISO 10993). In particular, established test systems for biocompatibility and cytotoxicity of long-term biomaterials have limited use and reliability when used to investigate degradable Mg alloys. Additionally, the results obtained in vitro are substantially different from in vivo observations. The physiological background and possible hypotheses will be elucidated and possible mechanism of in vivo corrosion of Mg alloys will be discussed. Several approaches to simulate the in vivo conditions on the laboratory scale have been investigated in the literature so far. These approaches will be presented and critically reviewed. This chapter will summarize the latest achievements and comment on the selection and use, test methods and the approaches to develop and produce Mg alloys that are intended to perform clinically with an appropriate host response.