  We study the low-energy effective theory in N=2 super Yang-Mills theories by
microscopic and exact approaches. We calculate the one-instanton correction to
the prepotential for any simple Lie group from the microscopic approach. We
also study the Picard-Fuchs equations and their solutions in the semi-
classical regime for classical gauge groups with rank r \leq 3. We find that
for gauge groups G=A_r, B_r, C_r (r \leq 3) the microscopic results agree with
those from the exact solutions.
