  We study total and partial supersymmetry breaking by freely acting orbifolds,
or equivalently by Scherk-Schwarz compactifications, in type I string theory.
In particular, we describe a four-dimensional chiral compactification with
spontaneously broken N=1 supersymmetry, some models with partial $N=4\to N=2$
and $N=4\to N=1$ supersymmetry breaking and their heterotic and M-theory duals.
A generic feature of these models is that in the gravitational sector and in
the spectrum of D-branes parallel to the breaking coordinate, all mass
splittings are proportional to the compactification scale, while global
(extended) supersymmetry remains unbroken at tree level for the massless
excitations of D-branes transverse to the breaking direction.
