  Debris disks, comprised of planetsimal belts and the dust and gas produced by
their mutual collisions, are the longest-lived phase of circumstellar disks.
Typically much fainter in emission than protoplanetary disks, debris disks can
be found in associations in which some members still host protoplanetary disks
rich in gas as well as around main sequence stars of all ages. There are even
classes of debris disks around AGB stars and white dwarfs. Typically, these
disks have been studied through dust emission though an increasing number of
young disks are now found to host some gas which may be remnants of the
protoplanetary disks or second-generation gas. The ngVLA will have a particular
niche in the study of the dust population of these disks, since the size
distribution of the dust can be derived from the spectral index of the spectral
energy distribution from the far-infrared to the centimetre. With the longest
lever arms provided by ngVLA, the size distribution can be characterized,
testing models of collisional evolution. The ngVLA could also be capable of
measuring the quantities of HI and OH emission associated with the disks. HI
and OH are by-products of the dissociation of water, which is expected to be a
primary component of outgassed material from cometary collisions. A probe of
the water content of extrasolar systems has important implications for the
delivery of water to terrestrial, potentially Earth-like planets.
