  I present in detail the case for regarding black hole thermodynamics as
having a statistical-mechanical explanation in exact parallel with the
statistical-mechanical explanation believed to underly the thermodynamics of
other systems. (Here I presume that black holes are indeed thermodynamic
systems in the fullest sense; I review the evidence for \emph{that} conclusion
in the prequel to this paper.) I focus on three lines of argument: (i)
zero-loop and one-loop calculations in quantum general relativity understood as
a quantum field theory, using the path-integral formalism; (ii) calculations in
string theory of the leading-order terms, higher-derivative corrections, and
quantum corrections, in the black hole entropy formula for extremal and
near-extremal black holes; (iii) recovery of the qualitative and (in some
cases) quantitative structure of black hole statistical mechanics via the
AdS/CFT correspondence. In each case I briefly review the content of, and
arguments for, the form of quantum gravity being used (effective field theory;
string theory; AdS/CFT) at a (relatively) introductory level: the paper is
aimed at students and non-specialists and does not presume advanced knowledge
of quantum gravity.. My conclusion is that the evidence for black hole
statistical mechanics is as solid as we could reasonably expect it to be in the
absence of a directly-empirically-verified theory of quantum gravity.
