The Summer Institute on High Energy Physics was the second of this kind organized at Louvain. Four years ago we had already decided to organize a Summer Institute. The first one was con­ ceived in 1970, at Kiev, by D. Speiser, J. Weyers, and G. Zweig, and thanks to a NATO grant took place from August 20th to Septem­ ber 15th 1971, at Louvain in the Groot Begijnhof. All lectures were directed toward one subject: duality. The lecturers were R. Brout (ULB - Bruxelles), D. Fairlie (University of Durham), F. Gilman (SLAC - Stanford), D. Horn (University of Tel Aviv), J. Mandula (Caltech - Pasadena), C. Michael (CERN - Geneva), J. Rosner (University of Minnesota), C. Schmidt (CERN - Geneva), J. Veneziano (The Weizmann Institute), J. Weyers (UCL - Louvain and CERN - Geneva), and G. Zweig (Caltech - Pasadena). The direc­ tion was in the hands of F. Cerulus (KUL - Louvain), R. Rodenberg (Technische Hochschule, Aachen), D. Speiser (UCL - Louvain), and J. Weyers (CERN - Geneva). Unfortunately it was not possible to publish the lecture notes for that Institute. The second Summer Institute on Elementary Particle Physics took place from August 12th to August 25th 1973, again in Louvain. It was initiated in Chicago, in 1972, by F. Halzen (University of Wisconsin) and J. Weyers (UCL - Louvain and CERN - Geneva). Lecturers included R. Carlitz (University of Chicago), F. Gilman (SLAC - Stanford), F. Halzen (University of Wisconsin), D.