 In light of the notorious difficulty of constructing a theory that incorporates and marries quantum mechanics and general relativity—a theory of quantum gravity—it may come as a surprise to learn that there is a consistent, rigorous theory of quantum fields posed on the background of a classical relativistic spacetime. (Wald 1994 is a standard text on the subject; Jacobson [2003, in  OIR]  gives a less rigorous overview, discussing possible relations to proposed theories of quantum gravity; Wald [2006b, in  OIR]  gives a synoptic history of the technical aspects of the entire subject, and an exposition of the advances in the field subsequent to the publication of Wald 1994; and Hollands and Wald 2015 provides a technically sophisticated overview of the most recent results.) Quantum field theory on curved spacetime, however, differs from standard quantum field theory (set on the flat Minkowski spacetime of special relativity) in one profound respect, that difference ramifying into every part of the theory: a generic relativistic spacetime has no group of symmetries comparable to the Poincaré Group for special relativity. There is correspondingly no distinguished vacuum state and no natural notion of a particle. This means, for instance, that one cannot employ many familiar and useful techniques from standard quantum field theory, and one must take care in the use of most of the others. 