  Reverberation mapping is a proven technique that is used to measure the size
of the broad emission-line region and central black hole mass in active
galactic nuclei. More ambitious reverberation mapping programs that are well
within the capabilities of Hubble Space Telescope could allow us to determine
the nature and flow of line-emitting gas in active nuclei and to assess
accurately the systematic uncertainties in reverberation-based black hole mass
measurements.
