  To derive black hole thermodynamics in any quantum theory of gravity, one
must introduce constraints that ensure that a black hole is actually present.
For a large class of black holes, the imposition of such ``horizon
constraints'' allows the use of conformal field theory methods to compute the
density of states, reproducing the correct Bekenstein-Hawking entropy in a
nearly model-independent manner. This approach may explain the ``universality''
of black hole entropy, the fact that many inequivalent descriptions of quantum
states all seem to give the same thermodynamic predictions. It also suggests an
elegant picture of the relevant degrees of freedom, as Goldstone-boson-like
excitations arising from symmetry breaking by a conformal anomaly induced by
the horizon constraints.
