  Supermassive black holes are nowadays believed to reside in most local
galaxies. Accretion of gas and black hole mergers play a fundamental role in
determining the two parameters defining a black hole: mass and spin. I briefly
review here some of the physical processes that are conducive to the evolution
of the massive black hole population. I'll discuss black hole formation
processes that are likely to place at early cosmic epochs, and how massive
black hole evolve in a hierarchical Universe. The mass of the black holes that
we detect today in nearby galaxy has mostly been accumulated by accretion of
gas. While black hole--black hole mergers do not contribute substantially to
the final mass of massive black holes, they influence the occupancy of galaxy
centers by black hole, owing to the chance of merging black holes being kicked
from their dwellings due to the gravitational recoil. Similarly, accretion
leaves a deeper imprint on the distribution of black hole spins than black hole
mergers do. The differences in accretion histories for black holes hosted in
elliptical or disc galaxies may reflect on different spin distributions.
