 Research on persistent luminescent materials has focused mainly on EU2-plus doped compounds. By now, the number of known persistent phosphors has increased over 200. Over 80% of these phosphors are based on intrinsic host defects, transition metals, manganese, chromium, copper, etc., or rare earths. This article was authored by Philippe F. Smet, Kuhn Van Denny Kaut, and Dirk Pellman.