 Finally, the evolution of black holes is apparently in conflict with standard quantum evolution, for such evolution rules out the sort of increase in entropy that seems to be required when black holes are present. Indeed, as purely gravitational entities with striking quantum properties, what we know about black holes lies at the heart of and guides many attempts to formulate a theory of quantum gravity. This has led to a debate over what seemingly fundamental physical principles are likely to be preserved in, or violated by, a full quantum theory of gravity. 