This volume contains the written versions of invited lectures and abstracts of seminars presented at the 26th "Universitatswochen fiir Kernphysik" (Uni versity nuclear physics weeks) in Schladming, Austria, in February 1987. Again the generous support of our sponsors, the Austrian Ministry of Sci ence and Research, the Styrian government and others, made it possible to invite expert lecturers. The meeting was organized in honour of Prof. Dr. th Walter Thirring in connection with his 60 birthday. In choosing the topics for the lectures we have tried to cover a good many of the areas in which...