 Attempts to quantize general relativity in the canonical framework proceed by turning the canonical variables into operators on an appropriate state space (e.g., the space of square-integrable functions over three-metrics), and dealing somehow with the constraints. When quantizing a theory with constraints, there are two possible approaches. The approach usually adopted in gauge theories is to deal with the constraints before quantization, so that only true degrees of freedom are promoted to operators when passing to the quantum theory. There are a variety of ways of doing this so-called ‘gauge fixing’, but they all involve removing the extra degrees of freedom by imposing some special conditions. In general relativity, fixing a gauge is tantamount to specifying a particular coordinate system with respect to which the “physical” data is described (spatial coordinates) and with respect to which it evolves (time coordinate). This is difficult already at the classical level, since the utility and, moreover, the very tractability of any particular gauge generally depends on the properties of the solution to the equations, which of course is what one is trying to find in the first place. But in the quantum theory, one is faced with the additional concern that the resulting theory may well not be independent of the choice of gauge. This is closely related to the problem of identifying true, gauge-invariant observables in the classical theory (Torre 2005, in the Other Internet Resources section).