Quantum Field Theory assumes that the curvature of space is zero. However, combining QFT with GR is possible, in such a way that matter is described by QFT, but gravity is described by GR. This theory can provide insights in the theory of Quantum Gravity, although gravity is treated classically. Wald continues to explain why it is not straightforward to extend QFT to curved space, as certain symmetries are not conserved in a curved space, and QFT in curved space does not comply with the interpretation of the vacuum and particle excitations and raises problems in defining the total energy as a positive quantity.
Wald continues to explain the algebraic approach to overcome these difficulties, and finishes with some key results that have followed from this approach.