Semiconductor spintronics aims to develop novel sensor, memory and logic devices by manipulating the spin states of carriers in semiconducting materials. This talk will focus on electrical spin injection into semiconductors, which is a prerequisite for spintronics and, in particular, on tunnel based spin injectors that are potentially operable above room temperature. The spin polarization of the electron current within the semiconductor is detected by measuring the circular polarization of the electroluminescence from a quantum well light emitting diode structure. The temperature and bias dependence of the electroluminescence polarization provides insight into the spin relaxation mechanisms within the semiconductor heterostructure.