Extended red emission: observational constraints for models

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.