This volume contains the proceedings of the International Winter School on Electronic Properties of High Temperature Superconductors, which was held March 3-10, 1990, in Kirchberg (Tyrol), Austria. This course was a sequel to three meetings on the subject of conducting polymers held there one, three and five years previously. The new topic was taken up since many of the scientists working on conducting polymers are now also actively interested in high-Tc superconductors, a research area that continues to develop at a remarkable pace. There have been numerous specialist conferences on high-Tc superconductors since their discovery by Bednorz and Muller in 1986. The Kirchberg meetings are intended as a school where experienced scientists from universities, research laboratories, and industry have the opportunity to discuss their most recent results, and where young scientists and students can learn about the present status of research in this field. The 1990 winter school was again organized in cooperation with the Bundesministerium fur Wissenschaft in Austria and with the Bundesministerium fur Forschung und Forschung und Technologie in the Federal Republic of Germany. The scientific interest was focused on the electronic structure of high-Tc superconductors and related compounds. At the beginning of each session, tutorial contributions were presented as an introduction for non-specialists. These were followed by research contributions elucidating the current state of our know ledge about high-Tc superconductors.