The unique feature ofelectron irradiations in compari­son to ions and neutrons is that they create defects in very low-energy recoil events. As a consequence, nearly all FPs are produced in isolation. This has been of foremost importance in developing our understanding of radiation damage, as it made studies of defect crea­tion mechanisms as well as the fundamental properties of FPs possible. Recall that the properties of vacancies and vacancy clusters, for example, formation and migra­tion energies, stacking fault energies, etc., could be determined from quenching studies. It is not possible, however, to quench in interstitials in metals. Very little was therefore known about this intrinsic defect prior to about 1955 when irradiation experiments became widely employed. In this section, we highlight some of the key findings derived from these past studies.