The concept of “active tundish slag” aims definitely at improving of steel cleanliness by tailoring a proper tundish slag. In this project, active tundish metallurgy was investigated and assessed. Computation of phase diagrams was used as an approach to understand properties and behavior of tundish slags in casting conditions. Eventual reactions between steel and slag in the tundish were examined by equilibrium calculations. Interfacial phenomena controlling inclusion removal and dissolution into tundish slag were investigated by applying several experimental techniques. Dissolution of alumina and spinel inclusions into slag were studied by model experiments applying CLSM and DHTT methods. Viscosity of slags was measured at different temperatures. Interfacial tension between steel and slag was measured by drop weight method for several slag/steel systems. The study showed the complexity and multi-disciplinarity in designing slags which can efficiently absorb macro- and micro-inclusions from different steels and at the same time act as protective cover against reoxidation and thermal losses. The final conclusion was that slag optimization is a challenging issue on which favorable metallurgical and thermal properties should be combined with long-lasting effect during sequential casting operation.