Differences between observed and calculated values was already well established in crystallography. From there it was, in retrospect, only a small step to refrain from using the integrated intensities as observed values but to use the actual measured profile intensities obtained by step scanning the powder diagram. The Rietveld method was first reported at the I.U.Cr. congress in Moscow in 1966. However, it was not until 1975, when it was also applied to X-ray diffraction, that it became widely accepted. Nowadays its use is no longer confined to elastic neutron powder diffraction, but to all diffraction techniques producing complex diffraction diagrams.