  Debris disks are optically thin, almost gas-free dusty disks observed around
a significant fraction of main-sequence stars older than about 10 Myr. Since
the circumstellar dust is short-lived, the very existence of these disks is
considered as evidence that dust-producing planetesimals are still present in
mature systems, in which planets have formed - or failed to form - a long time
ago. It is inferred that these planetesimals orbit their host stars at asteroid
to Kuiper-belt distances and continually supply fresh dust through mutual
collisions. This review outlines observational techniques and results on debris
disks, summarizes their essential physics and theoretical models, and then
places them into the general context of planetary systems, uncovering
interrelations between the disks, dust parent bodies, and planets. It is shown
that debris disks can serve as tracers of planetesimals and planets and shed
light on the planetesimal and planet formation processes that operated in these
systems in the past.
