  Debris disks are dusty, gas-poor disks around main sequence stars (Backman &
Paresce 1993; Lagrange, Backman & Artymowicz 2000; Zuckerman 2001).
Micron-sized dust grains are inferred to exist in these systems from
measurements of their thermal emission at infrared through millimeter
wavelengths. The estimated lifetimes for circumstellar dust grains due to
sublimation, radiation and corpuscular stellar wind effects are typically
significantly smaller than the estimated ages for the stellar systems,
suggesting that the grains are replenished from a reservoir, such as
sublimation of comets or collisions between parent bodies. Since the color
temperature for the excess emission is typically Tgr ~ 110 - 120 K, similar to
that expected for small grains in the Kuiper Belt, these objects are believe to
be generated by collisions between parent bodies analogous to Kuiper Belt
objects in our solar system; however, a handful of systems possess warm dust,
with Tgr > 300 K, at temperatures similar to the terrestrial planets. We
describe the physical characteristics of debris disks, the processes that
remove dust from disks, and the evidence for the presence of planets in debris
disks. We also summarize observations of infalling comets toward beta Pictoris
and measurements of bulk gas in debris disks.
