This paper by C.J. Isham provides an introduction to the problem of time in quantum gravity, which arises from the fundamental conflict between the concept of time used in quantum theory and its role in diffeomorphism-invariant theories like general relativity. The paper discusses three main categories of approaches to resolving this problem: (I) identifying time before quantizing, (II) identifying time after quantizing, and (III) approaches where time plays no fundamental role. Ten specific schemes are discussed, including the internal Schrödinger interpretation, matter clocks and reference fluids, unimodular gravity, the Klein-Gordon interpretation, third quantisation, the semi-classical approximation, decoherence of WKB solutions, the naive Schrödinger interpretation, the conditional probability interpretation, consistent histories, and the frozen formalism. The paper also covers the canonical decomposition of general relativity and addresses technical problems associated with these approaches, such as ultraviolet divergence and the interpretation of time in quantum gravity.This paper by C.J. Isham provides an introduction to the problem of time in quantum gravity, which arises from the fundamental conflict between the concept of time used in quantum theory and its role in diffeomorphism-invariant theories like general relativity. The paper discusses three main categories of approaches to resolving this problem: (I) identifying time before quantizing, (II) identifying time after quantizing, and (III) approaches where time plays no fundamental role. Ten specific schemes are discussed, including the internal Schrödinger interpretation, matter clocks and reference fluids, unimodular gravity, the Klein-Gordon interpretation, third quantisation, the semi-classical approximation, decoherence of WKB solutions, the naive Schrödinger interpretation, the conditional probability interpretation, consistent histories, and the frozen formalism. The paper also covers the canonical decomposition of general relativity and addresses technical problems associated with these approaches, such as ultraviolet divergence and the interpretation of time in quantum gravity.