  We discuss constraints on the assembly history of supermassive black holes
from the observed remnant black holes in nearby galaxies and from the emission
caused by accretion onto these black holes. We also summarize the results of a
specific model for the evolution of galaxies and their central black holes
which traces their hierachical build-up in CDM-like cosmogonies. The model
assumes (i) that black holes, ellipticals and starburts form during major
mergers of galaxies (ii) that the gas fraction in galaxies decreases with
decreasing redshift (iii) that the optical bright phase of a QSO lasts for
about 10^7 years. The model succesfully reproduces the evolution of cold gas as
traced by damped damped Lyman alpha systems, the evolution of optically bright
QSOs, the remnant black hole mass distribution and the host galaxy luminosities
of QSOs.
