As for many of the metal producing processes, the chemical system of ferromanganese production is quite complex. First, it involves a series of elements like Mn, Fe, Si, Ca, Mg, O and C. In addition it also covers a large temperature range, from less than 100 to more than 1600 °C area where both the solid, liquid and gas species must be taken into consideration. In the first analyses of a process, the thermodynamic equilibrium is considered. However, in the beginning of the 1990’s this was not known for more than 3 component slag systems. Hence, the last 25 years a tremendous effort has been performed in developing equilibrium data, kinetic data and reaction mechanisms for the production of ferromanganese production. This paper will go through the development within these areas the last 25 years.