  We study the vacuum structure and dyon spectrum of N=2 supersymmetric gauge
theory in four dimensions, with gauge group SU(2). The theory turns out to have
remarkably rich and physical properties which can nonetheless be described
precisely; exact formulas can be obtained, for instance, for electron and dyon
masses and the metric on the moduli space of vacua. The description involves a
version of Olive-Montonen electric-magnetic duality. The ``strongly coupled''
vacuum turns out to be a weakly coupled theory of monopoles, and with a
suitable perturbation confinement is described by monopole condensation.
