The greatest challenge in fundamental physics attempts to reconcile quantum mechanics and general relativity in a theory of "quantum gravity." This volume collects classic and original contributions from leading experts in both fields for a provocative discussion of the subject. It contains accessible introductions to the main and less-well-known known approaches to quantum gravity. It includes exciting topics such as the fate of spacetime in various theories, the so-called "problem of time" in canonical quantum gravity, black hole thermodynamics, and the relationship between the interpretation of quantum theory and quantum gravity.