Quantitative X-ray microanalysis requires the use of many fundamental constants related to the interaction of the electron beam with the sample. The current state of our knowledge of such constants in the particular areas of electron stopping power, X-ray ionization cross-sections, X-ray fluorescence yield, and the electron backscattering yield, is examined. It is found that, in every case, the quality and quantity of data available is poor, and that there are major gaps remaining to be filled.