  Our current understanding of interstellar dust is summarized at an
introductory level. Submicron-sized interstellar dust grains absorb and scatter
light, and reradiate the absorbed energy in the infrared. The grain population
spans a range of sizes, from molecules containing only tens of atoms, to
particles containing 10^{10} atoms. Most of the grain mass appears to be due to
two types of solid, in approximately equal amounts: amorphous silicate mineral,
and carbonaceous material. Approximately 2/3 of the interstellar carbon in
diffuse clouds is in solid form.
  Interstellar grains are important for many reasons: they obscure our view (in
a wavelength-dependent manner); the infrared emission from dust provides a
valuable probe of dense regions; and because dust grains play a central role in
interstellar chemistry, gas dynamics, and heating and cooling of interstellar
gas.
