The "instanton liquid"

We review the theory of interacting topological fluctuations in the ground state of quantum gauge theories, the so called "instanton liquid". First we outline some known phenomenological facts, both coming from "real" experiments (for QCD) and from the lattice data. Then we describe interaction of instantons and the statistical mechanics of their ensemble for the SU(2) gauge theory. The very essential role of the light quarks is considered using numerical experiments. One of the main conclusions is that instantons induce chiral symmetry breaking in vacuum, but as they are suppressed (e.g. by the nonzero temperature) this symmetry is restored. Phase transition is found to be very strong and of the first order.