Quantum field theory methods are necessary for strongly nonlinear problems of statistical physics. Here one deals with systems with the infinite number of degrees of freedom, as far as in the quantum field theory. The theory of fully developed turbulence is an example of a problem of that type. The well-known Navier-Stokes equation demonstrates the lost of stability of the solution at large velocities. Here the interacting eddies are born in liquid, these affect essentially the mean velocity of liquid. The problem to be solved is the description of probability distribution of the eddies.

With the essentially nonlinear processes one deals also while one investigates critical phenomena, waves propagation in critical media, Goldstone singularities, nonlinear plasma phenomena. The strong fluctuations and infinite correlation radius take places in all these systems.

Nonlinear Schwinger equations, functional Legendre transforms, quantum field perturbation expansions, quantum field renormalization group methods, instanton analysis and other methods of quantum field theory are used for the description of the systems mentioned.