Until recently, expressions derived for the probability distribution of the magnitudes of structure amplitudes have been valid only for the range from zero up to two or three times the average magnitude. Recent progress in obtaining results valid for larger structure amplitudes (up to unitary structure factors approaching unity) is reviewed; the methods employed include Fourier-series representations, steepest-descents approximations, and computer simulations. Except for the last, the results are valid only for the space group {ie335-02}, though those for the space groups belonging to the point group 2/m and possibly some other centro-symmetric groups of low symmetry are expected to behave similarly.
