The role of the tundish in the continuous casting process has evolved from that of a buffer between the ladle and mold to being a grade separator, an inclusion removal device and a metallurgical reactor. For both grade separation and inclusion removal, the flow patterns inside the tundish play an important role. The separation of non-metallic inclusions at the interface between the steel and the slag in the tundish is to a great extent controlled by interfacial phenomena in the steel-slag-inclusion system. One constant in the world of high-purity, low-residual and clean steels is the continual drive to reduce and control the frequency of the inclusions. For example, there are high-purity, low-residual clean steels such as IF- steel sheets for automobiles which require the absence of oxide inclusions with diameters larger than 100 micrometers. Great efforts have been made to optimize the flow and turbulence in the tundish through weirs, dams or other systems. Furthermore, a new system, the porous annular well block for injecting argon, has been developed to optimize the bubble size and distribution and hence to achieve a high efficiency in trapping inclusions and in decreasing the hydrogen content in steel. Consequently, the steel quality is further improved in the tundish. Tested with success in a seamless steel shop, the new system gives results in agreement with the water models.