active galactic nuclei (AGNs). Reverberation mapping (e.g., Blandford & McKee 1982) experiments can yield estimates of the radius of the broad-line region (BLR) from the lag between the variability in the AGN continuum and the corresponding variability in the broad permitted lines. With the BLR radius ([$R_{\mathrm{BLR}}$]{}) in hand and some measure of the BLR velocity dispersion ($v$), one can infer a virial mass for the BH, [$M_\mathrm{BH}$]{} $=v^2$[$R_{\mathrm{BLR}}$]{}/$G$ (Ho 1999; Wandel, Peterson, & Malkan 1999; Kaspi et al. 2000). Remarkably, this simple prescription yields BH masses that seem to be accurate to a factor of $\sim$3 when compared against the [$M_{\mathrm{BH}}-{\ensuremath{\sigma_{\ast}}}$]{} relation (Gebhardt et al. 2000b; Ferrarese et al. 2001; Nelson et al. 2004; Onken et al. 2004). The number of AGNs with BH masses derived from reverberation mapping is currently small ($35$), but [$M_\mathrm{BH}$]{} can be estimated indirectly from single-epoch spectra using an empirical correlation between [$R_{\mathrm{BLR}}$]{} and optical continuum luminosity that has been calibrated using the reverberation-mapped sample: [$R_{\mathrm{BLR}}$]{} $\propto L_{5100}^{0.7}$ (Kaspi [et al.]{} 2000), where [$L_{5100}$]{} = $\lambda L_{\lambda}$ at $\lambda$ = 5100 Å. From a single-epoch spectrum one can measure both $v$ and [$L_{5100}$]{}, and thereby infer [$M_\mathrm{BH}$]{}.

While virial estimates of BH masses have 