  Spin splittings in III-V materials and heterostructures are of interest
because of potential applications, mainly in spintronic devices. A necessary
condition for the existence of these spin splittings is the absence of
inversion symmetry. In bulk zincblende materials the inversion symmetry is
broken, giving rise to a small spin splitting. The much larger spin splitting
observed in quantum wells is normally attributed to the asymmetry of the
confining potential and explained on the basis of the Rashba effect. For
symmetrically confined wells, where the only source of asymmetry is that of the
underlying crystal potential, the confining potential strongly enhances the
spin splittings. This enhancement does not require the asymmetry of the
confining potential but depends on the interplay between the confinement and
the crystal potential. In this situation the behavior of the spin splittings is
consistent with the Dresselhaus contribution.
  In asymmetrically confined wells both Dresselhaus and Rashba terms
contribute.
  We present a general theory of the spin splittings of these structures based
on the group theory of diamond and zincblende heterostructures.
