  We propose explicit formulae for the integration measure on the moduli space
of charge-n ADHM multi-instantons in N=1 and N=2 supersymmetric gauge theories.
The form of this measure is fixed by its (super)symmetries as well as the
physical requirement of clustering in the limit of large spacetime separation
between instantons. We test our proposals against known expressions for n < 3.
Knowledge of the measure for all n allows us to revisit, and strengthen,
earlier N=2 results, chiefly: (1) For any number of flavors N_F, we provide a
closed formula for F_n, the n-instanton contribution to the Seiberg-Witten
prepotential, as a finite-dimensional collective coordinate integral. This
amounts to a solution, in quadratures, of the Seiberg-Witten models, without
appeal to electric-magnetic duality. (2) In the conformal case N_F=4, this
means reducing to quadratures the previously unknown finite renormalization
that relates the microscopic and effective coupling constants, \tau_{micro} and
\tau_{eff}. (3) Similar expressions are given for the 4-derivative/8-fermion
term in the gradient expansion of N=2 supersymmetric QCD.
