[en] Ionic solutions were among the first liquids to which x-ray diffraction was applied, and a large number of studies have been reported over the years. However, the interpretation of a single diffraction pattern is always difficult, often ambiguous, and never unique. This ambiguity of interpretation is greatly reduced if a solution is studied with several types of radiation (x-ray, neutron, electron), and a few such studies have been reported. The only currently feasible way of uniquely determining the correlations between water molecules and monatomic ions in solution is to vary the scattering factor of the ion; a simple difference measurement then yields the ion-water correlations. This has been done using the isotopic substitution method in neutron diffraction. It can also be done using synchrotron x-radiation and anomalous dispersion techniques. Diffraction studies of ion-water interactions have yielded detailed and unambiguous information for only a few concentrated solutions. 5 figures