Indirect characterization methods of steel cleanliness and non-metallic inclusions were summarized in the current chapter, including the detection of the total oxygen, nitrogen pickup, loss of dissolved aluminum of the steel, change of slag composition, refractory evaluation, etc., which were supplements to direct characterization methods and helped more accurately and comprehensively evaluate inclusions and cleanliness in steel. In addition, the performance of steel products could also be used to indirectly characterize non-metallic inclusions in steel.