[en] In order to understand and elucidate the role of the radiative and collisional processes in the plasma edge region of fusion reactors, it is essential to have available a detailed and quantitative knowledge on these elementary processes such as cross sections, reaction rate coefficients etc. Much effort has been recently devoted to the experimental determination of absolute partial and total electron impact ionization cross sections of molecules and radicals due to the ever increasing importance of these cross sections in the diagnostics of many applications such as low- and high-temperature plasma physics and chemistry, atmospheric physics and mass spectrometry. In addition supporting calculations have recently been developed in order to allow the analytic quantitative description of these cross section functions for modelling codes using a novel approach to the determination of electron induced appearance energies. In this review we will therefore discuss (i) experimental studies on electron impact ionisation of neutral molecules and electron impact ionisation of molecular ions including the measurement of ionization cross sections and appearance energies and the determination of kinetic energy release distribution for dissociative ionization and (ii) the theoretical determination of electron impact ionisation cross sections for the single ionisation of molecules and the multiple ionisation of atoms. (author)