AbstractNeutron diffraction has been of fundamental importance for the determination of the structure of high-temperature superconductors and for understanding the influence of structure on the critical temperature. This is because the new superconductors are heavy metal oxides; X rays are mainly scattered by the metal atoms, but thermal neutrons are scattered as strongly by oxygen, which is the atom of most interest in these materials. In fact, for the past 20 yr, neutron diffraction has been steadily gaining ground as an important technique in structural inorganic chemistry.