  The abundances of gas and dust (solids and complex molecules) in the
interstellar medium (ISM) as well as their composition and structures impact
practically all of astrophysics. Fundamental processes from star formation to
stellar winds to galaxy formation all scale with the number of metals. However,
significant uncertainties remain in both absolute and relative abundances, as
well as how these vary with environment, e.g., stellar photospheres versus the
interstellar medium (ISM). While UV, optical, IR, and radio studies have
considerably advanced our understanding of ISM gas and dust, they cannot
provide uniform results over the entire range of column densities needed. In
contrast, X-rays will penetrate gas and dust in the cold (3K) to hot
(100,000,000K) Universe over a wide range of column densities (log NH=20-24
cm^-2), imprinting spectral signatures that reflect the individual atoms which
make up the gas, molecule or solid. *X-rays therefore are a powerful and viable
resource for delving into a relatively unexplored regime for determining gas
abundances and dust properties such as composition, charge state, structure,
and quantity via absorption studies, and distribution via scattering halos.*
