The two outstanding features of the superconducting state are current transport without resistance and perfect diamagnetism. The sudden drop to zero resistance occurs at the critical temperature, which is a characteristic property of the superconductor in question. The discovery of high-temperature superconductivity in 1986 shifted the known critical temperatures from 23.2 K in metallic superconductors to values well above 30 K in copper-oxide (cuprate) high-temperature superconductors. A remarkably high critical temperature of 39 K has also been found in MgB2. Transition temperatures well above 77 K, providing the possibility of significantly reduced cooling cost, re-established the interest in applications of superconductivity in the power sector.