  We present a detailed study of various aspects of Spinor-Vector duality in
Heterotic string compactifications and expose its origin in terms of the
internal conformal field theory. In particular, we illustrate the main features
of the duality map by using simple toroidal orbifolds preserving N_4 = 1 and
N_4 = 2 spacetime supersymmetries in four dimensions. We explain how the
duality map arises in this context by turning on special values of the Wilson
lines around the compact cycles of the manifold. We argue that in models with
N_4 = 2 spacetime supersymmetry, the interpolation between the Spinor-Vector
dual vacua can be continuously realized. We trace the origin of the
Spinor-Vector duality map to the presence of underlying N = (2, 2) and N = (4,
4) SCFTs, and explicitly show that the induced spectral-flow in the twisted
sectors is responsible for the observed duality. The isomorphism between
current algebra representations gives rise to a number of chiral character
identities, reminiscent of the recently-discovered MSDS symmetry.
