Theoretical models of Thomson scattering in plasmas at frequencies which are not large relative to the plasma frequency rely, among other things, on an accurate description of three wave mixing in plasmas. Thomson scattering experiments making use of waves with frequencies in this range include fast ion diagnostics and various fluctuation and magnetic field diagnostics, but generally exclude conventional electron temperature diagnostics. The theory was originally developed using a traditional fluid approach. These results, which have been quoted widely, are, however, at variance with low temperature results obtained by Aamodt and Russell (1992) using a kinetic approach.