A requirement for the realization of semiconductor spintronics is the efficient injection, transport, control and detection of spin-polarized carriers in a semiconductor. Modified light emitting diodes known as spin-LEDs employ a magnetic contact to electrically inject spin-polarized carriers into a semiconductor diode structure. Spin-LEDs are the only spin-based structures where the combined efficiency of spin injection/transport/detection can be investigated reliably and in a model-independent way. The book reviews early studies of spin injection in spin-LED structures based on GaAs quantum wells. Spin injection from both a paramagnetic diluted magnetic semiconductor (ZnMnSe) as well as a ferromagnetic metal (Fe) are discussed. The discussion also contains an introduction on the physics of spin orientation in semiconductors along with a review of optical and magneto-optical techniques used for the investigation of spin-LEDs. The book is of interest to scientists in the field ofspin injection but it may also be useful as part of an introductory course on semiconductor spintronics.