  A semiclassical theory of spin dynamics and transport is formulated using the
Dirac electron model. This is done by constructing a wavepacket from the
positive-energy electron band, and studying its structure and center of mass
motion. The wavepacket has a minimal size equal to the Compton wavelength, and
has self-rotation about the average spin angular momentum, which gives rise to
the spin magnetic moment. Geometric gauge structure in the center of mass
motion provides a natural explanation of the spin-orbit coupling and various
Yafet terms. Applications of the spin-Hall and spin-Nernst effects are
discussed.
