  I summarize what we have learned about the nature of stars that ultimately
explode as core-collapse supernovae from the examination of images taken prior
to the explosion. By registering pre-supernova and post-supernova images,
usually taken at high resolution using either space-based optical detectors, or
ground-based infrared detectors equipped with laser guide star adaptive optics
systems, nearly three dozen core-collapse supernovae have now had the
properties of their progenitor stars either directly measured or (more
commonly) constrained by establishing upper limits on their luminosities. These
studies enable direct comparison with stellar evolution models that, in turn,
permit estimates of the progenitor stars' physical characteristics to be made.
I review progenitor characteristics (or constraints) inferred from this work
for each of the major core-collapse supernova types (II-Plateau, II-Linear,
IIb, IIn, Ib/c), with a particular focus on the analytical techniques used and
the processes through which conclusions have been drawn. Brief discussion of a
few individual events is also provided, including SN 2005gl, a type IIn
supernova that is shown to have had an extremely luminous -- and thus very
massive -- progenitor that exploded shortly after a violent, luminous blue
variable-like eruption phase, contrary to standard theoretical predictions.
