D-brane instanton effects in Type II orientifolds: local and global issues

We review how D-brane instantons can generate open string couplings of stringy hierarchy in the superpotential which violate global abelian symmetries and are therefore perturbatively forbidden. We discuss the main ingredients of this mechanism, focussing for concreteness on Euclidean $D2$-branes in Type IIA orientifold compactifications. Special emphasis is put on a careful analysis of instanton zero modes and a classification of situations leading to superpotential or higher fermionic F-terms. This includes the discussion of chiral and non-chiral instanton recombination, viewed as a multi-instanton effect. As phenomenological applications we discuss the generation of perturbatively forbidden Yukawa couplings in SU(5) GUT models and Majorana masses for right-handed neutrinos. Finally we analyse the mirror dual description of $D1$-instantons in Type I compactifications with $D9$-branes and stable holomorphic bundles. We present globally defined semi-realistic string vacua on an elliptically fibered Calabi-Yau realising the non-perturbative generation of Majorana masses.