  We review briefly direct and indirect methods of measuring the masses of
black holes in galactic nuclei, and then focus attention on supermassive black
holes in active nuclei, with special attention to results from reverberation
mapping and their limitations. We find that the intrinsic scatter in the
relationship between the AGN luminosity and the broad-line region size is very
small, ~0.11 dex, comparable to the uncertainties in the better reverberation
measurements. We also find that the relationship between reverberation-based
black hole masses and host-galaxy bulge luminosities also seems to have
surprisingly little intrinsic scatter, ~0.17 dex. We note, however, that there
are still potential systematics that could affect the overall mass calibration
at the level of a factor of a few.
