The latest publications regarding the development of technology to control inclusion compositions focusing on MgO·Al₂O₃ spinel inclusions were summarized in this review article. The problems caused by spinel inclusions, which affect practice as well as products were shown. The formation mechanism of MgO·Al₂O₃ spinel inclusions is secondly explained thermodynamically from the view points of chemistries of molten steels and slag compositions. Furthermore, crystallization behaviour of spinel was introduced. Countermeasures conducted in practices and laboratories were shown along with some problems still left that should be solved in the future.