Spin polarized charge carrier injection and transport in non-magnetic semiconductors is a key enabling mechanism for spin based electronic data processing. We present theoretical models to describe spin injection and spin transport in structures consisting of a ferromagnetic metal injector, a thin semiconductor layer, and a ferromagnetic metal collector. The semiconductors considered are conjugated polymers (e.g. PPV), small-molecule organic molecular crystals (e.g. pentacene), and inorganic semiconductors (e.g. silicon). In thermal equilibrium the charge carriers in these semiconductors are not spin polarized. Efficient spin injection requires that the semiconductor be driven far out of local thermal equilibrium. Since carrier mobilities (and other relevant parameters) in polymers, organic molecular crystals, and inorganic semiconductors differ by many orders of magnitude, their charge carrier injection characteristics differ significantly.