Gauge theory, which is the basis of all particle physics, is itself based on a few fundamental concepts, the consequences of which are often as beautiful as they are deep. In this short lecture course I shall try to give an introduction to these concepts, both from the physical and mathematical points of view. Then I shall show how these considerations lead to a nonabelian generalization of the well-known electric-magnetic duality in electromagnetism. I shall end by sketching some of the many consequences in quantum field theory that this duality engenders in particle physics. These are notes from a lecture course given in the Summer School on Geometric Methods in Quantum Field Theory, Villa de Leyva, Colombia, July 1999, as well as a series of graduate lectures given in Oxford in Trinity Term of 1999 and 2000.
