  We describe ongoing searches for intermediate-mass black holes with M_BH ~
100-10^5 M_sun. We review a range of search mechanisms, both dynamical and
those that rely on accretion signatures. We find that dynamical and accretion
signatures alike point to a high fraction of 10^9-10^10 M_sun galaxies hosting
black holes with M_BH<10^5 M_sun. In contrast, there are no solid detections of
black holes in globular clusters. There are few observational constraints on
black holes in any environment with M_BH ~ 100-10^4 M_sun. Considering low-mass
galaxies with dynamical black hole masses and constraining limits, we find that
the M_BH-sigma_* relation continues unbroken to M_BH~10^5 M_sun, albeit with
large scatter. We believe the scatter is at least partially driven by a broad
range in black hole mass, since the occupation fraction appears to be
relatively high in these galaxies. We fold the observed scaling relations with
our empirical limits on occupation fraction and the galaxy mass function to put
observational bounds on the black hole mass function in galaxy nuclei. We are
pessimistic that local demographic observations of galaxy nuclei alone could
constrain seeding mechanisms, although either high-redshift luminosity
functions or robust measurements of off-nuclear black holes could begin to
discriminate models.
