Gravity, at the classical level, is described by Einstein's field equations of general relativity, R μ μ ν ν − − 1 2 g μ μ ν ν R + g μ μ ν ν Λ Λ = 8 π π G c 4 T μ μ ν ν . These equations combine the spacetime geometry encoded in the metric g μ μ ν ν with the matter content comprised in the energy–momentum tensor T μ μ ν ν . The quantum nature of matter has been tested experimentally, for instance quantum electrodynamics is by now one of the most accurately confirmed theories in physics. For this reason quantization of gravity seems plausible, too. Unfortunately the quantization cannot be performed in the standard way (perturbative renormalization): Already a simple power-counting consideration signals the perturbative nonrenormalizability since the mass dimension of Newton's constant is − − 2 . The problem occurs as follows. According to the traditional point of view renormalization is implemented via the introduction of counterterms that should cancel divergent expressions appearing in loop integrals. Applying this method to gravity, however, the counterterms required to eliminate all divergences proliferate to an infinite number. As this inevitably leads to an infinite number of free parameters to be measured in experiments, the program is unlikely to have predictive power beyond its use as a low energy effective theory.