 In most, though not all, theories of quantum gravity, the gravitational field itself is also quantized. Since the contemporary theory of gravity, general relativity, describes gravitation as the curvature of spacetime by matter and energy, a quantization of gravity seemingly implies some sort of quantization of spacetime geometry: quantum spacetime. Insofar as all extant physical theories rely on a classical (non-quantum) spacetime background, this presents not only extreme technical difficulties, but also profound methodological and ontological challenges for the philosopher and the physicist. Though quantum gravity has been the subject of investigation by physicists for almost a century, philosophers have only just begun to investigate its philosophical implications.