 All approaches to canonical quantum gravity face the so-called “problem of time” in one form or another (Kuchař (1992) and Isham (1993) are still excellent reviews; Rickles, 2006, offers a more philosophical guide). The problem stems from the fact that in preserving the diffeomorphism-invariance of general relativity — depriving the coordinates of the background manifold of any physical meaning — the “slices” of spacetime one is considering inevitably include time, just as they include space. In the canonical formulation, the diffeomorphism invariance is reflected in the constraints, and the inclusion of what would ordinarily be a ‘time’ variable in the data is reflected in the existence of the super-Hamiltonian constraint. The difficulties presented by this latter constraint constitute the problem of time.