  We consider a subset of the physical processes that determine the spin j =
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 ~ 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 ~ 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 ~ 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.
