Photonic devices are becoming increasingly important in information and communication technologies. But attempts to integrate photonics with silicon-based microelectronics are hampered by the fact that silicon has an indirect band gap, which prevents efficient electron-photon energy conversion. Light-emitting silicon-based materials have been made using band-structure engineering of SiGe and SiC alloys and Si/Ge superlattices, and by exploiting quantum-confinement effects in nanoscale particles and crystallites(1-3). The discovery(4,5) that silicon can be etched electrochemically into a highly porous form that emits light with a high quantum yield has opened up the latter approach to intensive study(6-12). Here we report the fabrication, by molecular-beam epitaxy, of well-defined superlattices of silicon and SiO2, which emit visible light through photoluminescence. We show that this light emission can be explained in terms of quantum confinement of electrons in the two-dimensional silicon layers. These superlattice structures are robust and compatible with standard silicon technology.