Computer modelling of microstructural changes and the relationships between microstructure and mechanical properties of hot rolled steel products has been under active development in the research community. A new technology emerges from the modelling studies, attempting to predict the microstructural changes occurring during hot rolling and cooling of steels and to control their mechanical properties so that production is carried out under the optimum processing condition. This computer aided prediction and control technology is practiced using the mathematical models based on physical metallurgy. The model describes quantitatively the transformation behaviors during hot working and cooling, such as recrystallization, grain growth, precipitation and phase transition from austenite to ferrite. It also clarifies the relations existing among the processing condition, microstructure and the final mechanical properties. Various models of alloy steels as well as plain carbon steels have been developed for the last 10 years. It has been expected that the models would widely be applied to practice in the steel industry in the near future and make a great contribution to quality control and process optimization. Some prospective areas where the models are applied are guarantee of mechanical properties throughout the coil length, elimination of tensile tests, decrease in property variation, automatic resetting of processing conditions, save in alloys, and developing new process control models. However, in order to meet this expectation, there are many obstacles to be cleared with regard to model refining, understanding on physical metallurgy, sensor development, process control and quality design systems. It was also suggested that a close cooperation among researchers and engineers from different disciplines would be indispensable to accomplish the goal.