The phenomenon of superconductivity - after its discovery in metals such
as mercury, lead, zinc, etc. by Kamerlingh-Onnes in 19]] - has
attracted many scientists. Superconductivity was described in a very
satisfactory manner by the model proposed by Bardeen, Cooper and
Schrieffer, and by the extensions proposed by Abrikosov, Gorkov and
Eliashberg. Relations were established between superconductivity and the
fundamental properties of solids, resulting in a possible upper limit of
the critical temperature at about 23 K. The breakthrough that
revolutionized the field was made in 1986 by Bednorz and Muller with the
discovery of high-temperature superconductivity in layered copper-oxide
perovskites. Today the record in transition temperature is 133 K for a
Hg based cuprate system. The last decade has not only seen a revolution
in the size of the critical temperature, but also in the myriads of
research groups that entered the field. In addition, high-temperature
superconductivity became a real interdisciplinary topic and brought
together physicists, chemists and materials scientists who started to
investigate the new compounds with almost all the available experimental
techniques and theoretical methods. As a consequence we have witnessed
an avalanche of publications which has never occurred in any field of
science so far and which makes it difficult for the individual to be
thoroughly informed about the relevant results and trends. Neutron
scattering has outstanding properties in the elucidation of the basic
properties of high-temperature superconductors.