This chapter introduces the principle of how low-grade iron ores are upgraded to high-quality iron ore concentrates by magnetic separation. Magnetite is the most magnetic of all the naturally occurring minerals on earth, so low-intensity magnetic separators are used to upgrade magnetite ores. On the other hand, because oxidized iron ores like martite, hematite, specularite, limonite, and siderite are weakly magnetic, high-intensity magnetic separators and high-gradient magnetic separators are required to upgrade oxidized iron ores. Therefore, it is important to develop and optimize processing flow sheets according to the nature of iron ore to achieve both high recovery and high grade at a low cost. Three flow sheets for magnetite ores and seven flow sheets for oxidized iron ores separation are discussed.