III-V semiconductors near the metal insulator can be driven into a highly polarized state by spin injection from a ferromagnetic metal. I will review three aspects of these systems. In the first, I will cover recent progress in spin injection devices based on Heusler alloys, which provide for large (and tunable) spin polarizations. Second, I will discuss the dynamics of the coupled electron-nuclear spin system in these devices. It is in fact possible to measure the electron exchange field acting on the nuclei (the Knight field), and I will show how this provides an alternative means of measuring the electron spin polarization. Finally, I will review how one can measure extremely high spin polarizations using a technique analogous to the Seebeck effect.