  Probably the most fundamental characteristic of the quasar-AGN power house,
the mass of the central black hole, is the least well known. I review the three
main classes of mass estimation methods---broad emission-line kinematics, X-ray
variability and accretion-disk modeling, and the masses they give in terms of
the Eddington ratio, $L/L_{\rm Edd}$. The broad emission lines are probably the
best probe of the central mass. They provide mass estimates that suggest a
narrow spread for the Eddington ratio - 1--2 orders of magnitude over more than
six orders of magnitude in continuum luminosity, possibly indicating a
universal $M/L$ ratio for AGN. However, other methods give a larger spread and
possibly a luminosity- dependent Eddington ratio. Potential sources of errors
and bias are discussed.

Astrophysics