  We develop the general tools for model building with orientifolds, including
SS supersymmetry breaking. In this paper, we work out the general formulae of
the tadpole conditions for a class of non supersymmetric orientifold models of
type IIB string theory compactified on $T^6$, based on the general properties
of the orientifold group elements. By solving the tadpoles we obtain the
general anomaly free massless spectrum.
