  The exact solution of $N=2$ supersymmetric $SU(N)$ Yang-Mills theory is
studied in the framework of the Whitham hierarchies. The solution is identified
with a homogeneous solution of a Whitham hierarchy. This integrable hierarchy
(Whitham-Toda hierarchy) describes modulation of a quasi-periodic solution of
the (generalized) Toda lattice hierarchy associated with the hyperelliptic
curves over the quantum moduli space. The relation between the holomorphic
pre-potential of the low energy effective action and the $\tau$ function of the
(generalized) Toda lattice hierarchy is also clarified.
