Structural studies have played a key role in understanding the new oxide superconductors. Neutron powder diffraction techniques have been of particular importance because of the difficulty in synthesizing single crystals of the desired quality and compositions and because of the need to accurately determine the locations and site occupancies of the oxygen atoms. The electronic properties have been shown to depend in a critical way on the structural details of the Cu–O sublattices in these compounds. The La2-xMxCuO4-y (M=Ba, Sr, Ca,...) structure contains a two-dimensional square planar Cu–O sublattice with weak bonding between the layers. The YBa2Cu3O7-x structure contains both two-dimensional and one-dimensional Cu–O sublattices, commonly referred to as “planes” and “chains.” Closely-related non-superconducting phases which exhibit small differences in stoichiometry or symmetry have also been studied and have provided key insights into the close relationship between structure and superconductivity in these systems.