There is now good evidence that supermassive black holes reside at the
centers of most massive galaxies: the difference between active and
quiescent galaxies is due to differences in some combination of
accretion rate and radiative efficiency. In nearby quiescent and
active galaxies, black hole masses can be measured by modeling gas or
stellar dynamics, both of which require high angular resolution.
However, in active galaxies, masses can also be measured by
reverberation-mapping techniques where time resolution can be used as
a surrogate for angular resolution. I will discuss measurements of the
masses of the central black holes in galaxies and how these can be
used to anchor scaling relationships that allow estimates of the black
holes in distant quasars, with some emphasis on the current
limitations and intrinsic uncertainties of these techniques.