  During the past 30 years, research in general relativity has brought to light
strong hints of a very deep and fundamental relationship between gravitation,
thermodynamics, and quantum theory. The most striking indication of such a
relationship comes from black hole thermodynamics, where it appears that
certain laws of black hole mechanics are, in fact, simply the ordinary laws of
thermodynamics applied to a system containing a black hole. This article will
review the present status of black hole thermodynamics and will discuss some of
the related unresolved issues concerning gravitation, thermodynamics, and
quantum theory.
