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.

24 pages, plain latex, to appear in special ``Millennium Issue'' of Class. Quant. Grav