This Chapter is devoted to the modern theory and experimental investigations of the associative ionization (AI) reaction. The means of their further development are analyzed. Numerous applications in plasma chemistry, aeronomy, chemical physics of the upper atmosphere, and astrophysics have contributed to conducting this study. The threshold behavior of cross sections of the endothermic reaction AI is considered, and it is shown that the dependence of above-threshold energy E is a strictly linear in the quantum case which differs substantially from the law E^(3/2) resulting in a semiclassical theory. This result has a simple physical explanation because the matrix elements of scattering operator resulting from the tunneling effect are finite at E = 0. Comparison with the semiclassical theory has been made. A substantiation of the described possibility of elementary AI reaction dynamics in the diffusion approach is given. A detailed consideration of the applicability conditions of the «quantum chaos» theory fo