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 CuO
2 layer, which determines the occurrence of superconductivity. Close correlation between
Tc and hole concentration in nonstoichiometric YBa
2Cu
3O
7-Z and YBa
2Cu
4O
8 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 CO
3 group are presented.
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