Steel cleanliness was a topic of great concern in literatures, which included two aspects: the control of non-metallic inclusions in the steel, and the control of impurity elements in the steel. The existence of inclusions in the molten steel had a great influence on steel properties that needed to be strictly controlled. The history and progress of research on non-metallic inclusions was introduced which was traced back to over 100 years ago. The classification of inclusions was discussed according to the source, the morphology and distribution, and the deformability as well as the size. The effect of inclusions on steel properties was summarized.