The power of duality - Exact results in 4D susy field theory

Recently the vacuum structure of a large class of four-dimensional (supersy mmetric) quantum field theories was determined exactly. These theories exhi bit a wide range of interesting new physical phenomena. One of the main new insights is the role of "electric-magnetic duality." In its simplest form it describes the long distance behavior of some strongly coupled, and hence complicated, "electric theories" in terms of weakly coupled "magnetic theo ries." This understanding sheds new light on confinement and th Higgs mecha nism and uncovers new phases of four-dimensional gauge theories. We review these developments and speculate on the outlook.