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 many applications such as low- and high-temperature plasma physics and chemistry, atmospheric physics and mass spectrometry. In addition supporting calculations have been recently developed in order to allow the analytic quantitative description of these cross section functions for modeling codes. This review summarizes recent experimental and theoretical progress in this area of electron impact ionization including also a short description of (i) recent advances in the field of electron/ion collisions (i.e., kinetic energy release distributions in dissociative excitation reactions of molecular ions) and (ii) a novel approach to the determination of electron induced appearance energies. ©2000 American Institute of Physics.