The most fundamental aspects of single-electron transfer (SET) principles are presented. They are discussed according to different definitions used by expert practitioners and are.applied to SET living radical polymerization (SET-LRP) according to the definition of the division of organic chemistry of IUPAC that relies on principles elaborated by Taube, Eberson, Chanon, and Kochi. Additional definitions are also discussed to help clarify for the nonexpert contradictory literature reports. Subsequently, the principles and evolution of SET-LRP together with the methodologies currently available to practice it are discussed. It is expected that this Perspective will be able to help experts and non experts practice, develop, and invent new concepts and methodologies for SET-LRP to advance its status and the status of other living radical polymerization of the most precise living polymerization methods.