  The problem of time in quantum gravity occurs because `time' is taken to have
a different meaning in each of general relativity and ordinary quantum theory.
This incompatibility creates serious problems with trying to replace these two
branches of physics with a single framework in regimes in which neither quantum
theory nor general relativity can be neglected, such as in black holes or in
the very early universe. Strategies for resolving the Problem of Time have
evolved somewhat since Kuchar and Isham's well-known reviews from the early
90's. These come in the following divisions I) time before quantization, such
as hidden time or matter time. II) Time after quantization, such as emergent
semiclassical time. III) Timeless strategies of Type 1: naive Schrodinger
interpretation, conditional probabilities interpretation and various forms of
records theories, and Type 2 `Rovelli': in terms of evolving constants of the
motion, complete observables and partial observables. IV) I argue for histories
theories to be a separate class of strategy. Additionally, various combinations
of these strategies have begun to appear in the literature; I discuss a number
of such. Finally, I comment on loop quantum gravity, supergravity and
string/M-theory from the problem of time perspective.
