The unexpected and therefore really amazing discovery of J.G. Bednorz and K.A. MA1/4ller, that certain oxide compounds enter a superconductivity state at temperatures above 30 K, pushed research on superconductivity into the limelight of science in general in a way that seemed reserved only for high energy or particle physics. It may therefore be expected that this entire review would solely deal with superconductivity at high temperatures, i.e. above the boiling point of hydrogen. Any unexpected occurrence of superconductivity is, however, a challenge to scientists interested either in the physics of this phenomenon or in its materials science aspects. In this respect, the eighties have been quite revolutionary in the sense that, on various occasions, superconductivity was discovered in materials whose physical properties were not obviously favourable for adopting this ground state. Therefore, apart from emphasizing the topic of oxide superconductors, this collection of reprints also contains a selection of papers that deal with other subjects, such as coexistence of magnetic order and superconductivity, heavy electron and organic superconductors. This is all the more justified when we consider the fact that various aspects of superconductivity in high Tc oxide compounds are, or might be, connected with features that are also observed in these other materials. For nonspecialists who might be interested in this collection of reprints the Editor briefly reviews the possibilities for identifying superconductivity and discusses some special features of the superconducting state.

The unexpected and therefore really amazing discovery of J.G. Bednorz and K.A. Muller, that certain oxide compounds enter a superconductivity state at temperatures above 30 K, pushed research on superconductivity into the limelight of science in general in a way that seemed reserved only for high energy or particle physics. It may therefore be expected that this entire review would solely deal with superconductivity at high temperatures, i.e. above the boiling point of hydrogen. Any unexpected occurrence of superconductivity is, however, a challenge to scientists interested either in the physics of this phenomenon or in its materials science aspects. In this respect, the eighties have been quite revolutionary in the sense that, on various occasions, superconductivity was discovered in materials whose physical properties were not obviously favourable for adopting this ground state. Therefore, apart from emphasizing the topic of oxide superconductors, this collection of reprints also contains a selection of papers that deal with other subjects, such as coexistence of magnetic order and superconductivity, heavy electron and organic superconductors. This is all the more justified when we consider the fact that various aspects of superconductivity in high Tc oxide compounds are, or might be, connected with features that are also observed in these other materials. For nonspecialists who might be interested in this collection of reprints the Editor briefly reviews the possibilities for identifying superconductivity and discusses some special features of the superconducting state. "