Abstract
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.
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