  IR excesses of white dwarfs (WDs) can be used to diagnose the presence of
low-mass companions, planets, and circumstellar dust. Using different
combinations of wavelengths and WD temperatures, circumstellar dust at
different radial distances can be surveyed. The Spitzer Space Telescope has
been used to search for IR excesses of white dwarfs. Two types of circumstellar
dust disks have been found: (1) small disks around cool WDs with T_eff < 20,000
K, and (1) large disks around hot WDs with T_eff > 100,000 K. The small dust
disks are within the Roche limit, and are commonly accepted to have originated
from tidally crushed asteroids. The large dust disks, at tens of AU from the
central WDs, have been suggested to be produced by increased collisions among
Kuiper Belt-like objects. In this paper, we discuss Spitzer IRAC surveys of
small dust disks around cool WDs, a MIPS survey of large dust disks around hot
WDs, and an archival Spitzer survey of IR excesses of WDs.
