  We argue that for a large class of N=1 supersymmetric gauge theories the
effective superpotential as a function of the glueball chiral superfield is
exactly given by a summation of planar diagrams of the same gauge theory. This
perturbative computation reduces to a matrix model whose action is the
tree-level superpotential. For all models that can be embedded in string theory
we give a proof of this result, and we sketch an argument how to derive this
more generally directly in field theory. These results are obtained without
assuming any conjectured dualities and can be used as a systematic method to
compute instanton effects: the perturbative corrections up to n-th loop can be
used to compute up to n-instanton corrections. These techniques allow us to see
many non-perturbative effects, such as the Seiberg-Witten solutions of N=2
theories, the consequences of Montonen-Olive S-duality in N=1* and Seiberg-like
dualities for N=1 theories from a completely perturbative planar point of view
in the same gauge theory, without invoking a dual description.
