".... with the huge success of the quantum theory, starting especially with the Schrödinger equation in 1926, came a feeling among the leading physicists that mathematics should keep in the background or, as one person put it, `elegance is for tailors'. From the other side, mid-twentieth century mathematicians were not much more hospitable about intrusions of physics, as we can see, for instance, in Hardy's well known little essay. Walter was one of the first, in the post-war years, to try to put things back together." -- from the Foreword by Elliott Lieb This book contains Thirring's scientific contributions to mathematical physics, statistical physics, general relativity, quantum field theory, and elementary particle theory from 1950 onward. The order of the papers within the various sections is chronological and reflects the development of the fields during the second half of this century. In some cases, Thirring returned to problems decades later when the tools for their solution had ripened. Each section contains introductory comments by Thirring, outlining his motivation for the work at that time.