  This article reviews spin-dependent transport of carriers in homogenous
three-dimensional and two-dimensional semiconductors. We begin with a
discussion of optical orientation of electron spins, which allows both the
creation and detection of spin-polarized carriers in semiconductors. Then we
review non-equilibrium spin flow including spin drift and diffusion caused by
electric fields and concentration gradients. A controlled spin precession is
possible both in external magnetic fields and in effective magnetic fields due
to a broken inversion symmetry. Although the Coulomb interaction does not
couple to the spin degree of freedom, it affects the spin-dependent transport
via the spin Coulomb drag. In gyrotropic media, the optical creation of
spin-oriented electrons gives rise to spin photocurrents, which reverse their
direction when the radiation helicity is changed from left-handed to
right-handed. The reverse process is possible, too, i.e., an electric current
in a gyrotropic medium gives rise to a spin polarization in the bulk of the
sample.
