The results described in the previous sections provide a
remarkably compelling case that stationary black holes are
localized thermal equilibrium states of the quantum gravitational
field, and that the laws of black hole mechanics are simply the
ordinary laws of thermodynamics applied to a system containing a
black hole. Although no results on black hole thermodynamics have
been subject to any experimental or observational tests, the
theoretical foundation of black hole thermodynamics appears to be
sufficiently firm as to provide a solid basis for further
research and speculation on the nature of quantum gravitational
phenomena. In this section, I will briefly discuss two key
unresolved issues in black hole thermodynamics which may shed
considerable further light upon quantum gravitational physics.