Abstract: We will introduce the instanton solutions in the gauge theories and find a realization of them in string theory by building a model of D-branes and D-instantons in type IIB. We explain then how a new type of instantons called "exotic" or "stringy" instantons emerges from string theory. Stringy instantons correct the low-energy effective action but do not directly appear in the gauge field theories. We calculate the effect of multi-stringy-instantons in the prepotential of the N=2 super Yang-Mills with gauge group SU(2) and its generalization to SU(N)