In the past half century we have seen an explosive growth in the study of chemical reaction dynamics, spurred by advances in both experimental and theoretical techniques. Chemical processes are now measured on timescales as long as many years and as short as several femtoseconds, and in environments ranging from high vacuum isolated encounters to condensed phases at elevated pressures. This large variety of conditions has lead to the evolution of two branches of theoretical studies. On one hand, “bare” chemical reactions involving isolated molecular species are studied with regard to the effect of initial conditions and of molecular parameters associated with the relevant potential surface(s). On the other, the study of chemical reactions in high-pressure gases and in condensed phases is strongly associated with the issue of environmental effects. Here the bare chemical process is assumed to be well understood, and the focus is on the way it is modified by the interaction with the environment.