An important advance towards the construction of such a model was recently made by Zubko, Dwek, and Arendt (2003, ZDA03) who, for the first time, included the average interstellar extinction, the diffuse infrared emission, and the interstellar abundances as explicit constraints in the construction of models consisting of astrophysical dust particles without any predetermined functional form for their grain size distribution. The results showed the existence of many distinct dust models that satisfy a basic set of observational constraints.

X-ray halos, produced primarily by small angle scattering off large dust particles along the line of sight to bright X-ray sources, probe dust properties largely inaccessible at other wavelengths. In this contribution we briefly review the microscopic physics and the macroscopic effects that determine the intensity and spatial profile of X-ray halos. Focusing on the X-ray halo around the bright source GX 13+1, we show that halo observations can be used to discriminate between the different, currently viable, ZDA03 dust models. X-ray halos may therefore be an essential constraint that need to be explicitly included in all future dust models.