Current and voltage applied to the magnetic nanopillars induce a spin injection and an accumulation of nonequilibrium charges in a nanosize magnetic cell and result a spin torque exerted on the magnetic moment. Using such torques, we may amplify a precession of magnetization and induct a magnetization switching. These phenomena provide new techniques to write information into tiny magnetic cells and to construct oscillators and rectifiers that are several tens of nanometers in size. In this chapter, spin injections, and current and voltage-induced spin torques in magnetic multilayers, which show giant magnetoresistance effect in current-perpendicular-to-plane (CPP-GMR) geometry, and magnetic tunneling junctions are described. Further, mechanisms of spin injection and voltage-induced magnetization switching and its high-speed observations are explained. Then, phenomena related to spin injection, namely, spin-transfer oscillation and the spin-torque diode effect, are described. Finally, applications related to the spin-injection technology are reviewed.