The methodology of phase diagrams and that of alloy design were reviewed. The construction of a thermodynamic database for multicomponent systems by the CALPHAD method are shown, where the first principles calculations play a significant role in the estimation of thermodynamic properties of metastable phases. Some examples of alloy design for advanced materials using phase diagrams are shown in Pb-free machinable steels, high strength steels with low density, superelastic Fe-based and Cu-based alloys, magnetic recording media of Co-Cr-based alloys and Co-based superalloys.