The purpose of this chapter is to present a hierarchic modelling approach to phase separation of ferrite during low temperature aging of stainless steels. This phenomenon is responsible for the so-called 475C embrittlement in ferrite-containing stainless steels and enabling predictive modelling of the underlying phase transformation is of high technical importance, since such steels are vital in e.g., power generation applications. The suggested modelling approach is adopted in a project within the research center Hero-m at KTH in Sweden and currently steps towards predictive modelling of multicomponent alloys are taken. The hierarchic modeling utilizes: first-principles calculations to build fundamental understanding and to evaluate thermodynamic, kinetic and elastic data; CALPHAD-type thermodynamics modelling to build thermodynamic and kinetic databases; and phase-field modelling to simulate the nanostructure evolution. Experimental measurements are also an integrated part of the modelling approach. We demonstrate that the continuum modelling treatments in the literature are insufficient, but also that the suggested approach solving the non-linear Cahn-Hilliard equation, considering initial concentration fluctuations, and using accurate thermodynamic and kinetic input data, provide a viable path.