  We are living in a dusty universe: dust is ubiquitously seen in a wide
variety of astrophysical environments, ranging from circumstellar envelopes
around cool red giants to supernova ejecta, from diffuse and dense interstellar
clouds and star-forming regions to debris disks around main-sequence stars,
from comets to interplanetary space to distant galaxies and quasars. In this
review I focus on interstellar grains, with particular emphasis on the
extinction (absorption plus scattering) and emission properties of cold
submicron-sized ``classical'' grains which, in thermal equilibrium with the
ambient interstellar radiation field, obtain a steady-state temperature of
\~15--25K, warm nano-sized (or smaller) "ultrasmall" grains which are, upon
absorption of an energetic photon, transiently heated to temperatures as high
as a few hundred to over 1000K, and the possible existence of a population of
very cold (< 10K) dust. Whether dust grains can really get down to
"temperature" less than the 2.7K cosmic microwave background radiation
temperature will also be discussed. The robustness of the
silicate-graphite-PAHs interstellar dust model is demonstrated by showing that
the infrared emission predicted from this model closely matches that observed
for the Milky Way, the Small Magellanic Cloud, and the ringed Sb galaxy
NGC7331.
