Magnesium and its alloys are promising materials for biodegradable implant applications. The degradation behavior strongly depends on the environment of exposure, which can strongly differ for in vitro and in vivo environments. The presentation will describe frequently observed large differences in the corrosion rate of Mg in different types of simulated body fluids and discuss critical factors of biocorrosion of Mg alloys. Electrochemical measurements and surface analysis indicate a strikingly different behavior in SBF (simulated body fluid) as compared with cell culture medium (DMEM), drastically lower corrosion rates are observed in DMEM than in SBF. In both electrolytes the surface becomes covered by Ca-phosphate-rich layers, which mainly differ in their morphology (degree of porosity). In addition, effects of solution flow and temperature on corrosion behavior of Mg in biological environments will be discussed.