$A_0$ condensate in QCD

A survey devoted to $A_0$-condensate in gauge theories at high temperature is presented. Both the theoretical foundations of the spontaneously generated condensate and known methods of its calculation are discussed. As most important consequence the SU(N) global symmetry breakdown is investigated in details. Influence of $A_0$ on matter fields is studied in different aspects. Some new results concerning this subject are reported as well.