 The discovery of senior L204EU2+, DI3+, in 1996 marked a major breakthrough in the field of persistent luminescence. Since then, numerous other materials have been developed that exhibit similar properties. Despite the progress made since then, however, there is still much to be understood about the underlying mechanisms responsible for the persistence of these materials. This review provides an overview of the current state of knowledge regarding the preparation and properties of persistent luminescent materials, focusing specifically on those based on EU2+, emission. It also discusses the proposed models and mechanisms that have been suggested to explain the bright afterglow observed in various compounds. This article was authored by Cohn Van Dennykout, Philippe F. Smit and Dirk Pillman.