With rapid enough cooling, a metallic melt can change like other matter into an amorphous solid, i.e., metallic glass, through a glass transition. Metallic glass has attracted much attention since its discovery because of its excellent and novel physical, chemical and mechanical properties. In the past, metallic glass could only be produced in thin strips due to its low glass formation ability, and thus its application was restricted. By studying the relation-ships between the alloy components, undercooling and stability of the melt, and glass formation ability, bulk metallic glass of sizes from millimeters to several centimeters can be obtained in many alloy systems by convenient methods and with low cooling rates. This has laid a foundation for the wide application of metallic glass.