  We propose a robust supergravity model of dynamical supersymmetry breaking
and gauge mediation, and a natural embedding in non-perturbative string theory
with D-branes. A chiral field (and its mirror) charged under "anomalous" U(1)'s
acts as a Polonyi field whose hierarchical Polonyi-term can be generated by
string instantons. Further quartic superpotential terms arise naturally as a
tree-level decoupling effect of massive string states. A robust supersymmetry
breaking minimum allows for gauge mediation with soft masses at the TeV scale,
which we realise for a globally consistent SU(5) GUT model of Type I string
theory, with a D1-instanton inducing the Polonyi term.
