  To explain black hole thermodynamics in quantum gravity, one must introduce
constraints to ensure that a black hole is actually present. I show that for a
large class of black holes, such ``horizon constraints'' allow the use of
conformal field theory techniques to compute the density of states, reproducing
the Bekenstein-Hawking entropy in a nearly model-independent manner. One
standard string theory approach to black hole entropy arises as a special case,
lending support to the claim that the mechanism may be ``universal.'' I argue
that the relevant degrees of freedom are Goldstone-boson-like excitations
arising from the weak breaking of symmetry by the constraints.
