  We study the spectrum of stable BPS and non-BPS D-branes in Z_2 x Z_2
orientifolds for all choices of discrete torsion between the orbifold and
orientifold generators. We compute the torsion K-theory charges in these D=4,
N=1 orientifold models directly from worldsheet conformal field theory, and
compare with the K-theory constraints obtained indirectly using D-brane probes.
The K-theory torsion charges derived here provide non-trivial constraints on
string model building. We also discuss regions of stability for non-BPS
D-branes in these examples.
