AbstractWhereas the fundamental properties of all metallic elements are covered systematically and comprehensively in Chap. 4, this chapter treats magnesium which is applied as both a base and an alloying element of metallic materials (Sects. 5.1, 5.2). According to common usage, the chapter is subdivided into treatments of metallic materials, such as melting and casting, as well as heat treatment (Sect. 5.3), joining (Sect. 5.4) and corrosion behavior (Sect. 5.5). Recent developments are covered in the final Sect. 5.6. Magnesium is the lightest structural metal with a density of 1.74 g cm−3. It is produced by 2 basic processes. One is the electrolysis of fused anhydrous magnesium chloride (MgCl2) derived from magnesite, brine, or seawater, and recently from serpentine ores. The other one is the thermal reduction of magnesium oxide (MgO) by ferrosilicon derived from carbonate ores [5.1]. The use of primary Mg is shown in Fig. 5.1. Only one third is used for structural parts, mainly for castings, while another third is used as an alloying element in Al alloys.