  Supermassive black holes are located at the center of most, if not all,
massive galaxies. They follow close correlations with global properties of
their host galaxies (scaling relations), and are thought to play a crucial role
in galaxy evolution. Yet, we lack a complete understanding of fundamental
aspects of their growth across cosmic time. In particular, we still do not
understand: (1) whether black holes or their host galaxies grow faster and (2)
what is the maximum mass that black holes can reach. The high angular
resolution capability and sensitivity of 30-m class telescopes will
revolutionize our understanding of the extreme end of the black hole and galaxy
mass scale. With such facilities, we will be able to dynamically measure masses
of the largest black holes and characterize galaxy properties out to redshift
$z \sim 1.5$. Together with the evolution of black hole-galaxy scaling
relations since $z \sim 1.5$, the maximum mass black hole will shed light on
the main channels of black hole growth.
