In studying the thermodynamics and phase stability of Mg alloys, one is often confronted with the lack of accurate, quantitative experimental data. This deficiency can be partially rectified via first-principles calculations based on density functional theory. In this paper, we will illustrate the utility of first-principles energetics for Mg alloys using three case studies (i) formation energies of ordered/disordered solid phases; (ii) solute–vacancy binding energies in Mg; (iii) point defect formation energies of β-Mg 17Al 12. These first-principles calculations can provide highly accurate thermodynamic and kinetic information for Mg alloys.