Open Strings and Supersymmetry Breaking

We review several mechanisms for supersymmetry breaking in orientifold models. In particular, we focus on non-supersymmetric open-string realisations that correspond to consistent flat-space solutions of the classical equations of motion. In these models, the one-loop vacuum energy is typically fixed by the size of the compact extra dimensions, and can thus be tuned to extremely small values if enough extra dimensions are large.