  The principle of S-duality is used to incorporate gaugino condensates into
effective supergravity (superstring) Lagrangians. We discuss two
implementations of S-duality which differ in the way the coupling constant is
transformed. Both solve the problem of the runaway dilaton and lead to
satisfactory supersymmetry breaking in models with a {\em single} gaugino
condensate. The breakdown of supersymmetry is intimately related to a
nontrivial transformation of the condensate under T-duality.
