  (abridged) Thus far our impressions regarding the evolutionary time scales
for young circumstellar disks have been based on small number statistics. Over
the past decade, however, in addition to precision study of individual
star/disk systems, substantial observational effort has been invested in
obtaining less detailed data on large numbers of objects in young star
clusters. This has resulted in a plethora of information now enabling
statistical studies of disk evolutionary diagnostics. Along an ordinate one can
measure disk presence or strength through indicators such as ultraviolet/blue
excess or spectroscopic emission lines tracing accretion, infrared excess
tracing dust, or millimeter flux measuring mass. Along an abscissa one can
track stellar age.
  While bulk trends in disk indicators versus age are evident, observational
errors affecting both axes, combined with systematic errors in our
understanding of stellar ages, both cloud and bias any such trends Thus
detailed understanding of the physical processes involved in disk dissipation
and of the relevant time scales remains elusive. Nevertheless, a clear effect
in current data that is unlikely to be altered by data analysis improvements is
the dispersion in disk lifetimes. The age at which evidence for inner accretion
disks ceases to be apparent for the vast majority (90%) of stars is in the
range 3-8 Myr. More distant, terrestrial zone dust is traced by mid-infrared
emission where sufficient sensitivity and uniform data collection are only now
being realized with data return from the Spitzer Space Telescope.
