Phase transformations in steels are currently treated with empirical approaches which make use of the vast amount of existing knowledge and experience. The aim of this paper is to show that more fundamental approaches are complementary, and mandatory in order to arrive at predictive capabilities. Such approaches have to take into account the local conditions on a microscopic level such as inhomogeneities, element distributions, driving forces, thermodynamic properties of the phases. With this sophistication it becomes possible to treat complex situations such as the long term stability of microstructures under service conditions, giving room for new optimizations.