  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.
