  In this review, we describe in detail two important spin-transport phenomena:
the extrinsic spin-Hall effect (coming from spin-orbit interactions between
electrons and impurities) and the spin-Coulomb drag. The interplay of these two
phenomena is analyzed. In particular, we discuss the influence of scattering
between electrons with opposite spins on the spin current and the spin
accumulation produced by the spin-Hall effect. Future challenges and open
questions are briefly discussed.
