This volume contains the written versions of invited lectures presented at the 29th "Internationale Universitatswochen fiir Kernphysik" in Schladming, Aus tria, in March 1990. The generous support of our sponsors, the Austrian Ministry of Science and Research, the Government of Styria, and others, made it possible to invite expert lecturers. In choosing the topics of the course we have tried to select some of the currently most fiercely debated aspects of quantum field theory. It is a pleasure for us to thank all the speakers for their excellent presentations and their efforts in preparing the lecture notes. After the school the lecture notes were revised by the authors and partly rewritten ~n '!EX. We are also indebted to Mrs. Neuhold for the careful typing of those notes which we did not receive in '!EX. Graz, Austria H. Mitter July 1990 W. Schweiger Contents An Introduction to Integrable Models and Conformal Field Theory By H. Grosse (With 6 Figures) .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1 1. Introduction ............................................. . 1 1.1 Continuous Integrable Models .......................... . 1 1.2 "Solvable" Models of Statistical Physics ................. . 2 1.3 The Yang-Baxter Relation ............................. . 3 1.4 Braids and I(nots .................................... . 3 1.5 Confonnal Field Theory d = 2 ......................... . 3 2. Integrable Continuum Models - The Inverse Scattering Method - Solitons .................... . 4 2.1 A General Scheme for Solving (Linear) Problems ......... . 4 2.2 The Direct Step ...................................... . 6 2.3 The Inverse Step ..................................... .