This volume covers up-to-date ideas associated with the studies of high Tc superconductivity. Diverse theoretical points of view on the nature of this phenomenon are presented in the book. Experimental works discuss the results obtained in the studies of Bi and Tl superconductivity compounds as well as the thorough investigation of the different properties of 123 (YBaCu) compounds. Several articles are devoted to SQUIDs functioning at nitrogen temperatures and their application in research work.