Composition of interstellar gas and dust

The composition of interstellar dust is normally inferred indirectly, from analyses of gas-phase abundances and an assumed total interstellar medium composition, providing data on the depletions of elements from the gas. Additional information comes from infrared observations of solid-state absorption bands (usually applicable only to dense clouds), from X-ray absorption and scattering measurements, and from the analysis of components of interplanetary dust particles. This review is focused on issues related to interstellar gas-phase abundances and depletions, and the question of appropriate reference abundance standards. The paper includes an overview of gas-phase abundance measurements, a summary of depletions as derived from the adoption of solar or stellar reference abundances, comments on the impact of depletion measures on the inferred dust composition, and a discussion of alternatives to the adoption of solar or stellar reference abundances. The paper concludes with a description of new instruments, soon to be in operation, that should help to resolve some of the issues.