This volume contains the contributions to the INTERNATIONAL SUMMER INSTITUTE ON THEORETICAL PHYSICS 1980 held from September 1st to September 12th in Bad Honnef, Germany. This Institute was organized by Wuppertal University. It was the eleventh in a series of Summer Schools on particle physics carried out by German Universities. The Institute was aimed to review the present status of gauge theories in elementary particle physics, with emphasis both on the phenomenological and formal aspects. The first part of the volume covers the recent progress in the development of perturbative methods...