Superconductivity above the temperature of liquid nitrogen.

This is an invited tutorial review of high temperature superconductors for the IEEE Communications Society. The oxide superconductors are put into historical perspective and discussed with respect to their crystal structure and chemical composition. Their physical properties are reviewed in comparison with the best intermetallic superconductors. Fabrication techniques and possible practical applications, as well as some open questions about the mechanism for superconductivity are treated briefly.