We consider a subset of the physical processes that determine the spin j \equiv a / M of astrophysical black holes .
These include : ( 1 ) Initial conditions .
Recent models suggest that the collapse of supermassive stars are likely to produce black holes with j \sim 0.7. ( 2 ) Major mergers .
The outcome of a nearly equal mass black hole-black hole merger is not yet known , but we review the current best guesses and analytic bounds .
( 3 ) Minor mergers .
We recover the result of Blandford & Hughes that accretion of small companions with isotropically distributed orbital angular momenta results in spindown , with j \sim M ^ { -7 / 3 } .
( 4 ) Accretion .
We present new results from fully relativistic magnetohydrodynamic accretion simulations .
These show that , at least for one sequence of flow models , spin equilibrium ( dj / dt = 0 ) is reached for j \sim 0.9 , far less than the canonical value 0.998 of Thorne that was derived in the absence of MHD effects .
This equilibrium value may not apply to all accretion flows , particularly thin disks .
Nevertheless , it opens the possibility that black holes that have grown primarily through accretion are not maximally rotating .