  Black hole spins affect the efficiency of the "classical" accretion
processes, hence the radiative output from quasars. Spins also determine how
much energy is extractable from the hole itself. Recently it became clear that
massive black hole spins also affect the retention of black holes in galaxies,
be cause of the impulsive "gravitational recoil", up to thousands km/s, due to
anisotropic emission of gravitational waves at merger. I discuss here the
evolution of massive black hole spins along the cosmic history, due to the
combination of mergers and accretion events. I describe recent simulations of
accreting black holes in merger remnants, and discuss the implication for the
spins of black holes in quasars.
