The review is devoted to presentation of experimental and some theoretical results of the study of ionization processes in collisions of atomic particles of low energy. One of the main purposes of the review is to attempt to systematize the numerous results and methods of the experimental study of processes of formation of charged particles in collisions with energies below the characteristic ionization potentials (socalled chemi-ionization processes). The main attention in the review is devoted to studies carried out in recent years on the dynamics, energetics, and the electronic mechanism of processes involving particles in reliably identified states. Detailed discussions are given of questions of chemi-ionization with dissipation of electronically excited particles (Penning ionization), processes with transfer of an electron, and associative ionization of atomic particles of thermal energies. In conclusion some promising features of further studies of chemionization processes are discussed.