Ab initio methodologies provide, despite limitations that will be briefly commented on, fundamental insight on materials behavior and properties. This will be illustrated with a few ab initio-based predictions on phase stability, ordering trends, and thermodynamic properties. Examples will include the case of FeSi2, and of bcc-based transition metal alloys. In addition, ab initio output plays an important role in supplementing in two ways CALPHAD that is the most versatile and preferred method for assessing the thermodynamics of complex multi-component alloys: either by direct input of ab initio energetics in thermodynamic databases, or, more challenging, by assessing ab initio-based thermodynamics à la CALPHAD. These two applications will be briefly discussed in the context of phase diagram determination for selected transition metal and actinide-based alloys. Finally, a few comments on prospects in the alloy theory field, of critical importance for advancing our fundamental knowledge of materials performance, will conclude this presentation.