T he operation of the rapidly expanding range of electronic devices, such as personal computers and mobile phones, is primarily based on the control of electron charge in semiconductors. Although the tremendous progress in microfabrication technologies has accelerated the miniaturization of electronic devices, the size of devices will soon encounter the fundamental physical limits of that miniaturization. Further scale reduction beyond these limits will require a radical alteration of the concept of functional devices. Control of the spin degree of freedom of an electron has brought about a new era of integration in electronics over the last ten years, and research in the fi eld of 'spintronics' is currently being pursued extensively due to the potential of this approach for the development of a new direction in electronics. Th e fi eld of spintronics spreads beyond the traditional boundaries among research fi elds, leading to interdisciplinary research that spans magnetism, semiconductors, photonics and electronics.Since 1990, a number of spin-based device concepts have been proposed. Spin-based transistors [1][2][3][4][5], spin light-emitting diodes (spin-LEDs) [6,7] and spin transfer torque memories [8,9] are typical examples. One of the device concepts is depicted in Figure 1, where the device structure consists of a ferromagnetic source electrode, a highmobility semiconductor channel, a ferromagnetic drain electrode and a gate electrode. To operate a spin-based functional device, the primary issue is the effi cient injection of spin-polarized electrons from the source ferromagnet into the semiconductor channel across the interface. Th e electron spin is then manipulated by the application of a voltage via the gate electrode [10], and the spin orientation is detected using a ferromagnetic drain electrode in a spin fi eld-eff ect transistor (FET) or from the circular polarization of light emission in a spin-LED. Although the device concept is very simple and similar to that of a conventional FET, the implementation of spin-based devices is not that straightforward: the spin degree of freedom must be handled very delicately because the spin current rapidly depolarizes at the ferromagnet/semiconductor interface and the spin information will be lost within a time scale of several hundred picoseconds, even in a semiconductor [11,12]. Th is is in clear contrast to the conservation of electron charge. Th erefore, there are still obstacles to overcome in the implementation of such spintronic devices, and for this purpose a clear understanding of not only electron spin transport processes from a ferromagnet into a semiconductor, and vice versa, but also electron spin dynamics in semiconductors, is of fundamental and technological importance.Our focus in this article is on reviewing the current status of this subject with special emphasis on electrical and optical spin injection Spin-based electronics or 'spintronics' is a rapidly expanding research area that off ers the promise of surpassing the limits of conventiona...