Fe-Cr alloys are the base of ferritic steels, which are used in a wide range of high-temperature applications. Below 600°C, they undergo a coherent decomposition between Fe-rich (α) and Cr-rich (α’) phases. The kinetics of α-α’ phase separation has been extensively studied, because of its industrial relevance but also as a typical case of spinodal decomposition. However many open questions remains e.g. on the Cr solubility limit, on the composition of the phases during the α’ precipitation, and on the segregation and precipitation behaviors under irradiation. Most of them are related to the special magnetic properties of Fe-Cr alloys. We present an atomic scale modeling of phase separation in Fe-Cr alloys, using ab initio calculations and Monte Carlo simulations. It provides an atomic description of the α-α’ microstructure: the size, shape and composition of α and α’ domains are directly compared to experimental – especially tomographic atom probe – observations.