  One of the main themes in extragalactic astronomy for the next decade will be
the evolution of galaxies over cosmic time. Many future observatories,
including JWST, ALMA, GMT, TMT and E-ELT will intensively observe starlight
over a broad redshift range, out to the dawn of the modern Universe when the
first galaxies formed. It has, however, become clear that the properties and
evolution of galaxies are intimately linked to the growth of their central
black holes. Understanding the formation of galaxies, and their subsequent
evolution, will therefore be incomplete without similarly intensive
observations of the accretion light from supermassive black holes (SMBH) in
galactic nuclei. To make further progress, we need to chart the formation of
typical SMBH at z>6, and their subsequent growth over cosmic time, which is
most effectively achieved with X-ray observations. Recent technological
developments in X-ray optics and instrumentation now bring this within our
grasp, enabling capabilities fully matched to those expected from flagship
observatories at longer wavelengths.
