The present status of electron-density studies on high-Tc superconductors is reviewed within the X-ray diffraction method. Difference-Fourier maps of La2-xSrxCuO4, YBa2Cu3O7-x and Tl2CaBa2Cu2O8 have been reported so far. Since the superconductivity always appears associated with copper atoms in distorted perovskite structures, the electron-density distribution around the atoms is discussed on the electronic states. Although severe termination effect of Fourier series was observed in Tl2CaBa2Cu2O8, recent treatments on anharmonic thermal vibrations gradually make it possible to study the electron densities in a superconductive state below Tc.
