  The astrophysical processes that led to the formation of the first seed black
holes and to their growth into the supermassive variety that powers bright
quasars at redshift 6 are poorly understood. In standard LCDM hierarchical
cosmologies, the earliest massive holes (MBHs) likely formed at redshift z~15
at the centers of low-mass (M>5e5 solar masses) dark matter ``minihalos'', and
produced hard radiation by accretion. FUV/X-ray photons from such
``miniquasars'' may have permeated the universe more uniformly than EUV
radiation, reduced gas clumping, and changed the chemistry of primordial gas.
The role of accreting seed black holes in determining the thermal and
ionization state of the intergalactic medium depends on the amount of cold and
dense gas that forms and gets retained in protogalaxies after the formation of
the first stars. The highest resolution N-body simulation to date of Galactic
substructure shows that subhalos below the atomic cooling mass were very
inefficient at forming stars.
