A systematic presentation is given of the current state of experimental and theoretical studies of elementary processes involved in chemi-ionization by collision between atoms and molecules at thermal energies. General conclusions are presented about the rates of these processes, their importance in practical applications, and their effect on the macroscopic properties of the medium in which they take place. The review is intended for research workers and for postgraduate and graduate students specializing in plasma physics, atom-molecule collisions, and the interaction between radiation and matter.