 Though progress is being made in trying to at least draw observational consequences of loop quantum gravity, a theory of quantum gravity which arguably does make predictions (Amelino-Camelia, 2003, in the Other Internet Resources section below; D. Mattingly, 2005), it is remarkable that the most notable “test” of quantum theories of gravity imposed by the community to date involves a phenomenon which has never been observed, the so-called Hawking radiation from black holes. Based on earlier work of Bekenstein (1973) and others, Hawking (1974) predicted that black holes would radiate energy, and would do so in proportion to their gravitational “temperature,” which was in turn understood to be proportional to their mass, angular momentum, and charge. Associated with this temperature is an entropy (see the entry on  the philosophy of statistical mechanics),  and one would expect a theory of quantum gravity to allow one to calculate the entropy associated with a black hole of given mass, angular momentum, and charge, the entropy corresponding to the number of quantum (micro-)states of the gravitational field having the same mass, charge, and angular momentum. (See Unruh, 2001, and references therein.) In their own ways, string theory and loop quantum gravity have both passed the test of predicting an entropy for black holes which accords with Hawking’s calculation, using very different microscopic degrees of freedom. String theory gets the number right for a not-particularly-physically-realistic subset of black holes called near-extremal black holes, while loop quantum gravity gets it right for generic black holes, but only up to an overall constant. More recently, the causal set approach has also managed to derive the correct value. If the Hawking effect is real, then this consonance could be counted as evidence in favor of either or both/all theories.