Quantitative procedures in electron probe microanalysis (EPMA) require the knowledge of various atomic parameters, the most fundamental of which is the ionization cross section. A number of semi-empirical, approximate analytical formulas have been proposed to calculate the ionization cross section. The simplicity of these formulas makes them suitable for quantitative EPMA procedures. However, it is difficult to assess their reliability because of the lack of accurate experimental data. Indeed, inspection of currently available data reveals that they are still scarce for many elements and, when they are available, one usually finds significant discrepancies between data from different authors. Fortunately, the inaccuracies in the semi-empirical cross section formulas used in EPMA have only a small effect on the analytical results when standards are used. Nonetheless, in quantitative EPMA studies at low overvoltages or using standardless methods, the evaluated compositions largely depend on the adopted ionization cross sections and, therefore, knowledge of accurate ionization cross sections is a requisite for the development of improved quantification methods.