  I review scenarios for the assembly of supermassive black holes (MBHs) at the
center of galaxies that trace their hierarchical build-up far up in the dark
matter halo "merger tree". Monte Carlo realizations of the merger hierarchy in
a LCDM cosmology, coupled to semi-analytical recipes, are a powerful tool to
follow the merger history of halos and the dynamics and growth of the MBHs they
host. X-ray photons from miniquasars powered by intermediate-mass "seed" holes
may permeate the universe more uniformly than EUV radiation, make the
low-density diffuse intergalactic medium warm and weakly ionized prior to the
epoch of reionization breakthrough, and set an entropy floor. The spin
distribution of MBHs is determined by gas accretion, and is predicted to be
heavily skewed towards fast-rotating Kerr holes, to be already in place at
early epochs, and not to change significantly below redshift 5. Decaying MBH
binaries may shape the innermost central regions of galaxies and should be
detected in significant numbers by LISA.
