Abstract: Er-doped Si-based light emitting materials have important potential applications in light sources for optical communication and fiber amplifiers. Erbium doping has recently emerged as a promising route towards Si-based optoelectronic integration. In this paper the luminescence mechanism of Er-doped silicon and several factors hampering its industrial applications, as well as the role of O when co-doped with Er, are discussed. Several methods to increase the luminescence efficiency, various problems and new developments are reviewed.