This symposium has been concerned with the origin and evolution of the elements. The main points of the present paper are as follows:
The chemical composition of interstellar dust remains uncertain. PAH molecules are present, and a substantial fraction of the grain mass most likely consists of amorphous silicate, but precise compositional information still eludes us.
The grain size distribution is strongly constrained by observations of extinction, scaterring, and infrared emission, and extends from grains containing just tens of atoms to grains with radii a 0.3 µm.
Dust grains will drift through interstellar gas, and this transport process could produce local variations in the dust/gas ratio.
The dust mass required to account for interstellar extinction using homogeneous spherical grains exceeds that inferred from depletion studies (Jenkins 2003) by a factor ~ 1.5.
Dust grains could possibly deplete a significant fraction of interstellar D. This mechanism should be considered as a possible explanation for observed variations in the interstellar D/H ratio.
I thank Ed Jenkins and Todd Tripp for many valuable discussions, and Robert Lupton for availability of the SM software package. This research was supported in part by NSF grant AST-9988126.