In quantum field theory, the effective action Γ Γ is an analogue of the classical action functional S and depends on the fields of a given theory. It includes all quantum and thermal fluctuations. Variation of Γ Γ yields exact quantum field equations, for example for cosmology or the electrodynamics of superconductors. Mathematically, Γ Γ is the generating functional of the one-particle irreducible Feynman diagrams. Interesting physics, as propagators and effective couplings for interactions, can be straightforwardly extracted from it. In a generic interacting field theory the effective action Γ Γ , however, is difficult to obtain. FRG provides a practical tool to calculate Γ Γ employing the renormalization group concept.