Selectivity, energetic saving, and reduction of environmental impact associated with chemical processes have become main subjects of research in chemistry, the design of new routes and methods aimed at the chemical transformation of unwanted products is a very important issue that is increasingly addressed by researchers. This chapter reviews few concepts and methodologies that have been developed to get insights into reaction mechanisms. It explores two complementary ways to characterize and rationalize the reaction electronic flux (REF), a physical perspective in which the REF is expressed in terms of polarization and electron transfer phenomena and a chemical approach in which the REF is expressed in terms of the chemical reactivity descriptors, molecular hardness, and electrophilicity index. A closer view of the REF shows that electronic activity initiate with a tiny and broad positive peak which is mostly due to electronic transfer effects, although polarization effects are also present at that interval of the reaction coordinate.