  The results obtained by Seiberg and Witten for the low-energy Wilsonian
effective actions of N=2 supersymmetric theories with gauge group SU(2) are in
agreement with instanton computations carried out for winding numbers one and
two. This suggests that the instanton saddle point saturates the
non-perturbative contribution to the functional integral. A natural framework
in which corrections to this approximation are absent is given by the
topological field theory built out of the N=2 Super Yang-Mills theory. After
extending the standard construction of the Topological Yang-Mills theory to
encompass the case of a non-vanishing vacuum expectation value for the scalar
field, a BRST transformation is defined (as a supersymmetry plus a gauge
variation), which on the instanton moduli space is the exterior derivative. The
topological field theory approach makes the so-called "constrained instanton"
configurations and the instanton measure arise in a natural way. As a
consequence, instanton-dominated Green's functions in N=2 Super Yang-Mills can
be equivalently computed either using the constrained instanton method or
making reference to the topological twisted version of the theory. We
explicitly compute the instanton measure and the contribution to $u=<\Tr
\phi^2>$ for winding numbers one and two. We then show that each
non-perturbative contribution to the N=2 low-energy effective action can be
written as the integral of a total derivative of a function of the instanton
moduli. Only instanton configurations of zero conformal size contribute to this
result. Finally, the 8k-dimensional instanton moduli space is built using the
hyperkahler quotient procedure, which clarifies the geometrical meaning of our
approach.
