Non-metallic inclusions in steels like oxides, sulfides, nitrides, and complex inclusions affect the properties and performance of steel, such as machinability and mechanical strength. Therefore, control of inclusions in steel required accurate detection and characterization methods. In the current chapter, various methods used to directly detect and characterize non-metallic inclusions in the solid steel and in the molten steel were summarized, including the two-dimensional characterization, three-dimensional characterization, online evaluation, etc. The direct characterization methods were crucial for visualizing and identifying the inclusions, which was essential for improving the quality and performance of steel products. Overall, the direct detection and characterization of non-metallic inclusions were to ensure the desired properties and performance of steel materials.