A theory relating quantities in equilibrium and nonequilibrium statistical mechanics and microscopic and macroscopic quantities.

The underlying principle of the fluctuation-dissipation theorem is that a nonequilibrium state may have been reached either as a result of a random fluctuation or an external force and that the evolution towards equilibrium is the same in both cases.

It was first devised for electrical circuits with noise in 1928 by H.Nyquist; a general theorem in statistical mechanics was derived by H.B Callen and T.A Welton in 1951.