  The current picture painted by the observations of circumstellar dust at
white dwarfs, and the consequent atmospheric pollution, is of a surviving
planetary system. This chapter recounts in detail both the discovery and
empirical characterization of dust disks at single white dwarfs, including all
observational data available up to early 2011. Observations of the disks
themselves and the accreted heavy elements within the stellar photospheres are
consistent with tidally disrupted asteroid analogs. The observed chemistry
supports objects that condensed within the inner regions of main-sequence stars
and thus far appear similar in constitution to the terrestrial planets and
their building blocks.
