Recent development in mass spectrometry has far-reaching effects not only in physical and chemical studies of gaseous molecules and ions, but also in biological studies, including proteomics and metabolomics. In mass spectrometry, collisions are very important elementary processes in ionization, collision-induced dissociation, and mobility, whose methods are useful for the development of proteomics and metabolomics. In the present review, to understand the basis of collision, the concepts of center-of-mass system, laboratory system, cross section, kinetic energy release, and impact parameter are explained. The cross sections, which provide the magnitude of elementary reactions, depend on reaction species, collision energies, and reaction processes. I have attempted to present these dependences with the support of various examples.