  In the interstellar medium of the Milky Way, certain elements -- e.g., Mg,
Si, Al, Ca, Ti, Fe -- reside predominantly in interstellar dust grains. These
grains absorb, scatter, and emit electromagnetic radiation, heat the
interstellar medium by photoelectric emission, play a role in the ionization
balance of the gas, and catalyze the formation of molecules, particularly H2. I
review the state of our knowledge of the composition and sizes of interstellar
grains, including what we can learn from spectral features, luminescence,
scattering, infrared emission, and observed gas-phase depletions. The total
grain volume in dust models which reproduce interstellar extinction is
significantly greater than estimated from observed depletions.
  Dust grains might reduce the gas-phase D/H ratio, providing an alternative
mechanism to explain observed variations in the gas-phase D/H ratio in the
local interstellar medium. Transport in dust grains could cause elemental
abundances in newly-formed stars to differ from interstellar abundances.
