The total oxygen content (T.O) in steel was traditionally used as an index of steel cleanliness. In the current chapter, the cleanliness index Iclean for the control of oxide inclusions in steel was expressed by Iclean = x T.O + y(Al2O3). For Al-killed steels, the control of inclusions mainly emphasized the low content of T.O in the steel, and for Si-killed steels, the control of inclusions mainly emphasized that the content of Al2O3 in inclusions should be low. Topics on non-metallic inclusions in steel were discussed, including the detection of inclusions, deoxidation thermodynamics, calcium treatment, the deformation of inclusions, slag refining, nozzle clogging mechanism, the reaction mechanism between lining refractories and the molten steel, and the kinetic evolution of the size and composition of inclusions. Some misunderstandings on topics above were also listed and discussed, such as several misunderstandings on calcium treatment of the molten steel, the deformability of inclusions and its dependency on the melting point of inclusions, the complex deoxidation thermodynamics, and the neglect of the influence of alloy quality on non-metallic inclusions in steel.