Since the early 1990s, mathematical modelling has supported metallurgical research at MEFOS. In addition to thermodynamic calculations and trials in various scales, the commercial software PHOENICS has been used to describe the transport of mass and heat. Steady-state models, and transient models, have been developed for single-, 2- and 3-phase systems. In some applications, chemical reactions between the phases are also incorporated. Modelling activities have focused on the process route of steelmaking, including melting, secondary metallurgy and continuous casting. In this paper, highlights from some of these modeling activities are presented, including verification measurements. Process improvements are suggested, and some implications for treatment strategies and practices. Finally, future work and planned research are discussed.