The Mass-Luminosity Relation in AGN

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.