  Recently the vacuum structure of a large class of four dimensional
(supersymmetric) quantum field theories was determined exactly. These theories
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 terms of weakly coupled ``magnetic
theories.'' This understanding sheds new 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.
