The high angular resolution and sensitivity of the Chandra X-ray Observatory has yielded large numbers of faint X-ray sources with measured redshifts in the soft (0.5 - 2 keV) and hard (2 - 8 keV) energy bands. Many of these sources show few obvious optical signatures of active galactic nuclei (AGN). We use Chandra observations of the Hubble Deep Field-North region, together with other shallower surveys, to show that the number density of sources in the 10~(42) ergs s~(-1) to 10~(44) ergs s~(-1) luminosity range is rising, or is at least constant, with decreasing redshift. Broad-line AGN are the dominant population at higher luminosities, and these sources show the well-known rapid positive evolution with increasing redshift to z ～ 3. We argue that the dominant supermassive black hole formation has occurred at recent times in objects with low accretion mass flow rates rather than at earlier times in more X-ray luminous objects with high accretion mass flow rates. We also argue that there are too few moderate luminosity AGN at z = 5 - 6.5 to ionize the intergalactic medium.
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