  The non-perturbative dynamics of quantum field theories is studied using
theoretical tools inspired by string formalism. Two main lines are developed:
the analysis of stringy instantons in a class of four-dimensional N=2 gauge
theories and the holographic study of the minimal model for a strongly coupled
unbalanced superconductor.
  The field theory instanton calculus admits a natural and efficient
description in terms of D-brane models. In addition, the string viewpoint
offers the possibility of generalizing the ordinary instanton configurations.
Even though such generalized, or stringy, instantons would be absent in a
purely field-theoretical, low-energy treatment, we demonstrate that they do
alter the IR effective description of the brane dynamics by introducing
contributions related to the string scale. In the first part of this thesis we
compute explicitly the stringy instanton corrections to the effective
prepotential in a class of quiver gauge theories.
  In the second part of the thesis, we present a detailed analysis of the
minimal holographic setup yielding an effective description of a superconductor
with two Abelian currents. The model contains a scalar field whose condensation
produces a spontaneous symmetry breaking which describes the transition to a
superfluid phase. This system has important applications in both QCD and
condensed matter physics; moreover, it allows us to study mixed electric-spin
transport properties (i.e. spintronics) at strong coupling.
