Gaugino condensation, duality and supersymmetry breaking

The status of gaugino condensation in low-energy string theory is reviewed. Emphasis is given to the determination of the efective action below condensation scale in terms of the 2PI and Wilson actions. We illustrate how the different perturbative duality symmetries survive this simple nonperturbative phenomenon, providing evidence for the believe that these are exact nonperturbative symmetries of string theory. Consistency with T duality lifts the moduli degeneracy. The B_μν - axion duality also survives in a nontrivial way in which the degree of freedom corresponding to B_μν is replaced by a massive H_μνρ field but duality is preserved. S duality may also be implemented in this process. Some general problems of this mechanism are mentioned and the possible nonperturbative scenarios for supersymmetry breaking in string theory are discussed.