In looking ahead, we can see that important next steps in this field include:
Completing the mapping of the X-ray and optical luminosity functions for quasars and AGNs down to luminosities that include the bulk of the integrated radiation.
Refining methods for mass determinations and applying them to the full observed range of redshifts and luminosities.
Finding an observable spectral signature for accretion modes and efficiencies that will allow us to make reliable estimates of accretion rates. Are low-luminosity AGNs a result of low accretion rate, low efficiency, or low mass?
Determining the numbers of obscured sources and establishing the correlation between, for example, absorption in X-rays and UV/optical obscuration by dust. See if the results are consistent with deep radio and sub-mm observations.
Achieving a self-consistent fit of the population of observed, discrete X-ray sources with the overall intensity level and SED of the X-ray background.
If, in the end, we can match the observational data for AGNs over their entire redshift range to the local mass function of black holes in galaxies, we will have made a significant leap in our understanding of the coevolution of black holes and galaxies.
Acknowledgements. I am grateful to Eric Monier for assistance with the preparation of this article, especially the figures, and to David Weinberg, Marianne Vestergaard, Brad Peterson, and the anonymous referee for valuable comments on the first drafts. I thank the organizers for the opportunity to speak at the meeting.