  We derive a kinetic equation for the electrons moving on the surface of a
three-dimensional topological insulator. Due to the helical nature of the
excitations backward scattering is suppressed in the collision integral, and
the spin dynamics is entirely constrained by that of the charge. We further
analyze the tunneling between the helical and a conventional metal or
ferromagnet. We find that the tunnel resistance strongly depends on the angle
between the magnetization in the ferromagnet and the current in the helical
metal. A nonmagnetic layer on top of the helical metal amplifies the
current-induced spin polarization.
