C N Yang, one of the greatest physicists of the 20th Century, was awarded the Nobel Prize in 1957, jointly with T D Lee, for their investigation of the relationship (parity symmetry) between left- and right-handed states, leading to a discovery that astounded the world of physics — the nonconservation of parity by elementary particles and their reactions. With R L Mills, he created the concept of non-abelian gauge theories, the foundation of the modern description of elementary particles and forces. Professor Yang has worked on a wide range of subjects in physics, but his abiding interests have been symmetry principles, particle physics, and statistical mechanics.In 1999, a symposium was held at the State University of New York at Stony Brook to mark the retirement of C N Yang as Einstein Professor and Director of the Institute for Theoretical Physics, and to celebrate his many achievements. A noteworthy selection of the papers presented at the symposium appears in this invaluable volume in honor of Professor Yang.