  We consider four dimensional N=1 supersymmetric Type I compactifications on
toroidal orbifolds T^6/G. In particular, we focus on the Type I vacua which are
perturbative from the orientifold viewpoint, that is, on the compactifications
with well defined world-sheet expansion. The number of such models is rather
constrained. This allows us to study all such vacua. This, in particular,
involves considering compactifications with non-trivial NS-NS antisymmetric
tensor backgrounds. We derive massless spectra for these models, and also
compute superpotentials. We review the reasons responsible for such a limited
number of perturbative Type I compactifications on toroidal orbifolds (which
include Abelian as well as non-Abelian cases). As an aside, we generalized the
recent work on large N gauge theories from orientifolds to include a
non-Abelian orbifold. This also provides an important independent check for
perturbative consistency of the corresponding Type I compactification.
