Between 1911 and 1986, superconductivity was strictly a low-temperature phenomenon. The highest critical temperature Tc for any superconductor was 23.2 K for Nb3Ge. For any possible applications, the only useful refrigerants were liquid helium and liquid hydrogen. For much of this period, an understanding of the microscopic origin of superconductivity was lacking. Bardeen, Cooper, and Schrieffer published the BCS theory in 1957, and the more general Eliashberg theory soon followed. Theoretical predictions of the highest Tc one could hope for were not very useful for finding new materials with Tc above 23.2 K. In 1986, Bednorz and Müller discovered that La2CuO4 went superconducting when Ba substituted for some of the La. Later doping studies with Ca, Sr, and Ba showed that in this system Tc reached 38 K. Soon thereafter, Wu et al. found that suitably doped YBa2Cu3O7 had a Tc of 92 K, well above the boiling point of liquid nitrogen. In the intervening years several other classes of high-temperature superconductors were found. All of these had several properties in common. All contained one or more planes of Cu and O atoms per unit cell, all had structures which were related to the cubic perovskite structure, and all were related to a “parent compound” which was antiferromagnetic and insulating. Doping this parent compound (in one of several ways) produced a metal which was superconducting and, for some optimum doping, Tc could be very high, usually above the boiling point of liquid nitrogen.