  Observational constraints on the birth and early evolution of massive black
holes (BHs) come from two extreme regimes. At high redshift, quasars signal the
rapid growth of billion-solar-mass BHs and indicate that these objects began
remarkably heavy and/or accreted mass at rates above the Eddington limit. At
low redshift, the smallest nuclear BHs known are found in dwarf galaxies and
provide the most concrete limits on the mass of BH seeds. Here we review
current observational work in these fields that together are critical for our
understanding of the origin of massive BHs in the Universe.
