Crystal structures of high-Tc superconductors have been reviewed on the basis of neutron diffraction results. It is shown that the oxygen atoms play a crucial role in controlling hole concentration on the CuO2 layer, which determines the occurrence of superconductivity. Close correlation between Tc and hole concentration in nonstoichiometric YBa2Cu3O7-Z and YBa2Cu4O8 under pressure, oxygen location and occupancy in Tl-and Hg-based superconductors in connection with superconductivity and a novel structure in the newly-discovered superconductor containing the CO3 group are presented.
