Instantons in QCD Vacuum and Phase Transitions

In this lecture we review recent development in non-perturbative QCD related to instantons. The "instanton liquid model" was shown to reproduce not only the basic vacuum parameters (the condensates) but even hadronic correlators. New phenomenological information obtained from lattice simulations also confirm it. Meanwhile the model itself was developed into a self-consistent approach, allowing in practice keep 't Hooft interaction to all orders and also was generalized to non-zero temperatures and high densities. In both case chiral restoration phase transition is a structural transition from a relatively random instanton liquid to a gas of a strongly correlated instanton-anti-instantons, at finite T they are pairs (or "molecules") and at high density they are "polymers", or bound diquarks.