Rare-earth doped semiconductors hold great potential for a variety of optoelectronic applications, including lasers, LEDs and optical amplifiers. In fact, the field has grown rapidly over the past several years, with a clear switch in direction. The first book by this name was devoted to rare-earth doped II-VI and III-V semiconductors; more than half of the papers in this new volume are devoted to rare-earth doped silicon. This indicates that rare-earth doping of silicon is now seriously considered as a means to achieve silicon-based optoelectronic devices. In addition, new reports on rare-earth doped III-nitrides are also presented. Researchers from 14 countries come together in the volume to discuss current trends, highlight new developments and identify potential electronic and optoelectronic applications. Topics include: incorporation methods and properties; structural, electrical and optical properties; excitation mechanisms and electroluminescence and integration.