THE POWER OF DUALITY - EXACT RESULTS IN 4D SUSY FIELD-THEORY

Recently the vacuum structure of a large class of four-dimensional (su persymmetric) quantum field theories was determined exactly. These the ories exhibit 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 term s of weakly coupled ''magnetic theories.'' This understanding sheds ne w light on confinement and the Higgs mechanism and uncovers new phases of four-dimensional gauge theories. We review these developments and speculate on the outlook.