  On a manifold with boundary, the constraint algebra of general relativity may
acquire a central extension, which can be computed using covariant phase space
techniques. When the boundary is a (local) Killing horizon, a natural set of
boundary conditions leads to a Virasoro subalgebra with a calculable central
charge. Conformal field theory methods may then be used to determine the
density of states at the boundary. I consider a number of cases---black holes,
Rindler space, de Sitter space, Taub-NUT and Taub-Bolt spaces, and dilaton
gravity---and show that the resulting density of states yields the expected
Bekenstein-Hawking entropy. The statistical mechanics of black hole entropy may
thus be fixed by symmetry arguments, independent of details of quantum gravity.
