  The exact solution of N=2 supersymmetric QCD found by Seiberg and Witten
includes a prediction for all multi-instanton contributions to the low-energy
dynamics. In the case where the matter hypermultiplets are massless, the
leading non-perturbative contribution at weak coupling comes from the
two-instanton sector. We calculate this contribution from first principles
using the exact two-instanton solution of the self-dual Yang-Mills equation
found by Atiyah, Drinfeld, Hitchin and Manin. We find exact agreement with the
predictions of Seiberg and Witten for all numbers of flavours. We also confirm
their predictions for the case of massive hypermultiplets.
