  The origin and nature of Ultra-Luminous X-ray sources (ULXs) is a contentious
and controversial topic. There are ongoing debates about the masses of the
objects responsible, their sources of mass for accretion, and their relation to
stellar populations in galaxies. A new picture of these objects is proposed in
which they are intermediate-mass black holes confined to the disks of their
host galaxies and accreting from the interstellar medium. They are then
preferentially found in or near molecular clouds. This model correctly predicts
the shape of the observed luminosity function and requires only a very small
fraction of the baryonic mass of a galaxy to be in the form of
intermediate-mass black holes. Because the X-rays they produce strongly heat
nearby interstellar gas and because they move relatively rapidly in and out of
dense regions, ULXs are predicted to have brief episodes of high luminosity,
perhaps of order 10^5 yr in duration, but they may recur many times.
