We discuss spin-dependent charge carrier injection and transport in organic semiconductors from a device physics perspective. Structures considered have one or two ferromagnetic contacts to the organic semiconductor, and we discuss the conditions for which injection from the ferromagnetic contact may be strongly spin polarized. Electron tunneling from a ferromagnetic contact can have signicant spin dependence because the spatial part of the electron wavefunction is different for majority and minority spin states near the Fermi surface. By contrast, thermionic emission is not very spin dependent. Therefore, if charge injection is dominated by tunneling from a ferromagnetic contact, the injection can be strongly spin polarized, but if it is dominated by thermionic emission or another process that does not depend on spin, the injection will be only weakly spin polarized. We rst consider unipolar organic spin valve devices consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts into which one carrier type.