  Extended Red Emission (ERE) is a widely observed optical emission process,
present in a wide range of circumstellar and interstellar environments in the
Milky Way galaxy as well as other galaxies. Definitive identifications of the
ERE carriers and the ERE process are still a matter of debate. Numerous models
have been proposed in recent decades, often developed without consideration of
the growing body of observational constraints, which by now invalidate many of
these models. This review focuses on the most well-established observational
constraints which should help to delineate the way toward a generally accepted
explanation of the ERE and an understanding of its place in the radiation
physics of the interstellar medium.
