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 can 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. 
Carlo Angelantonj